Top-end rebuilding is the most frequent and costly service routine on two-stroke dirt bikes. Every year, dirt bike riders waste loads of money on top-end parts that didn?t need to be replaced, or make costly mistakes while performing repairs. This section will give you the dos and don?ts to easy top-end rebuilding, plus some tips that aren?t printed in your factory service manual.

Before You Start

Thoroughly wash your bike because dirt stuck to the underside of the top frame tube could break loose when servicing and fall into the engine! Use a stiff plastic brush and hot soapy water to clean off the grit and grime around the base of the cylinder, on the carburetor and intake boot, and especially underneath the top frame rail. Degreaser can be used on metal surfaces, but take care not to leave it on rubber or gasket surfaces.

Tools

You?ll need at least some 3/8-inch-drive metric sockets and box wrenches (open-end wrenches can round off the edges on the cylinder or head nuts, and shouldn?t be used for top-end rebuilding), a needle-nose pliers for removing circlips, and a gasket tool to scrape the old gaskets away. For soft tools, get some shop towels, aerosol oven cleaner, a Scotch-Brite pad, a locking agent such as Loctite, a gasket scraper, a brush, and a bucket of soapy water. Regarding measuring tools, you’ll need a compression tester, a feeler gauge, and a digital vernier caliper.

Compression Testing

A compression tester is a useful diagnostic tool, and readily available from Sears or auto parts stores. Buy the threaded type and make sure the kit comes with an adapter that matches the spark plug threads of your engine. Performing a compression test is simple. Start by removing the spark plug, thread in the adapter, and hold the throttle wide open and the kill button on. This will prevent any spark and enable the engine to draw in maximum airflow. Then kick-start the engine several times until the needle on the pressure gauge peaks. The pressure reading depends on two main factors; the compression ratio and the altitude at which the engine is being tested. The compression ratio will also depend on if the engine is equipped with exhaust valves and their condition. When the exhaust valves are in the closed position the compression ratio will be greater than if the valves are carbon-seized in the open position. The difference may yield a pressure reading 25 psi. The quality of compression testers varies greatly. The main thing that a compression tester can identify is a change in condition. Whenever you rebuild the top end, take a compression pressure reading and mark it down. When the pressure changes 20% check the condition of the piston and rings. Pistons usually last twice as long as rings.

Crankcase Pressure Testing

The crankcase of a two-stroke engine is sealed from the tranny. It’s important that the two crankshaft seals be in optimum condition. One side of the crankshaft uses a dry seal and the other a wet seal. The dry seal runs on the magneto side and the wet seal runs in oil on the tranny side. When the dry seal wears, the crankcase sucks in hot air, causing the mixture to run lean and overheat the engine. When the wet seal wears, the crankcase sucks in tranny oil, causing the engine run rich and eventually wet-foul the spark plug.

A crankcase pressure test involves the use of a vacuum pump with spark plug adapter, and rubber plugs to block off the intake and exhaust manifolds of the cylinder. The piston must be positioned at BDC to allow the transfer ports to be wide open linking the bore and the crankcase. The hand-pump produces vacuum pressure up to a standard setting of 5 psi. The normal bleed-down pressure loss is 1 psi per minute. Cylinders with complicated exhaust valve systems can be difficult to block-off air leaks, and harder to test. Crankcase pressure testing kits are available from Motion Pro.

If I suspect that an engine has an air leak in the crankcases, I do a visual test. Start by power washing the engine clean. Then remove the magneto cover. Spray the magneto clean with an aerosol can of brake cleaner. Make sure to use a non-chlorinated type of cleaner (green colored can). Now spray baby powder to all the suspect areas of the engine. Spray the powder on the crankcase around the magneto, at the crankcase seam line, the cylinder base, and the reed valve. Run the engine for a while, the white baby powder will highlight any fluid or air leaks on the engine. The baby powder test is much better than the alternative test of blowing raw propane gas at different areas of a running engine and listening for a change in the idle rpm. That is dangerous because it involves flammable gas and a hot engine with random electrical shorts.

Maintenance and Inspection

A thorough top-end rebuild requires removing the reed valve, cylinder head, and cylinder. You should tear down your top end periodically and inspect the reed valve, cylinder head, cylinder, piston, and so on. Use the following chart to determine when you should tear down your bike:

Displacement: 80cc 125cc 250cc 500cc

Tear down after: 5 hours 10 hours 20 hours 40 hours

Note that air-cooled bikes should be inspected more frequently. Also, you may want to inspect more often if you are riding in fine sand or lots of mud. When you tear down the engine, inspect each system and look for the following trouble signs.

Reed Valve

Check the reed petals for open gaps between the sealing surfaces. In time, the reed petals lose their spring tension, and the back-flow can cause a flat-spot in the throttle response. Stock nylon reeds tend to split at the edges on bikes that are constantly over-revved. Expert riders find that carbon fiber reeds last much longer.

Cylinder Head

Check the head at the edge of the chamber for erosion marks—a sign that the head gasket was leaking. If the head or top edge of the cylinder is eroded, it must be turned on a lathe to be resurfaced.

Cylinder

All cylinder bases use aligning (dowel. pins around two of the cylinder base studs. These pins are made of steel, and after heavy power washing, they get corroded. That makes it difficult to remove the cylinder from the crankcases. Never use a pry bar! That will damage the cylinder. Instead use a plastic mallet to hit upward on the sides of the cylinder at a 45-degree angle. Alternate from left to right sides so the cylinder lifts up evenly. After you remove the cylinder, stuff a shop towel into the open crankcases to prevent debris from entering the engine.

The Different Types of Steel-Lined and Plated Cylinders

There are two types of cylinder bores used on dirt bikes, steel or cast iron sleeves or ones with plating on the aluminum. Most dirt bikes made after 1989 have plated cylinders. You can check a cylinder with a magnet. If it sticks to the bore then it is a sleeve. If it doesn’t stick then it is plated. There are three types of plated cylinders, Kawasaki Electrofusion, hard-chrome, and nickel silicon carbide. There are several variations of the nickel silicon carbide process but the most common trade name is Nikasil. The nickel-based processes have many advantages over hard-chrome, Electrofusion, and sleeving. Nickel attracts oil and is an excellent carrier material for silicon carbide particles, a wear resistant material that carries the load of the piston. This material is electro-plated right on to the aluminum cylinder for the optimum thermal efficiency. Nickel can be honed with diamond stones which leave distinctive peaks and valley scratches in the cylinder wall which retain oil and provide a certain bearing ratio between the running surfaces of the bore. It’s possible to rebuild a plated cylinder by fitting it with a sleeve. However you can expect to pay more for bore maintenance over the life of the bike, and lose thermal efficiency and horsepower. Plated cylinders are harder and last longer than sleeved cylinders. Kawasaki cylinders with the original Electrofusion coating or hard-chromed cylinders can be repaired with nickel plating or sleeving. Steel or cast iron sleeves cannot be nickel plated unless they are separated from the aluminum cylinder. The reason is that the pretreatment for the plating would disintegrate the aluminum. There are four companies that replate cylinders in the USA. The average price to replate a cylinder is about $200.

The Piston

Some unfortunate guys do more damage replacing the piston than the actual wear on the piston! Remove the circlips with a small needle-nose pliers and throw them away. It is a common mistake to reuse circlips, but the cheap spring-steel wire clips will fatigue and break if you install them for a second time.

After removing the circlips, you have to remove the piston pin. Never use a hammer and punch to remove the pin. That will damage the connecting rod and needle bearings. Instead, use one of the pin-extractor tools available from your local franchised motorcycle shop. You can also grasp the piston with one hand and use a 3/8-inch socket extension to push the pin out with your other hand.

Too many people replace their pistons too often. The exact service interval for your bike depends on how hard the bike was run, for how many hours, the quality of the lubrication, and the amount of dirt or other debris in the intake air. Bikes that are run hard with dirty air filters may wear out pistons in only 6 hours, while bikes that are ridden easy with clean filters and adequate fuel octane may last 60 hours.

Measuring the Piston

The best thing to do is measure the piston with a caliper. Digital calipers cost about $100 at industrial tool companies such as Enco or Harbor Freight. A digital caliper is easy to use and gives accurate measurements on the piston diameter and cylinder bore. Measure the widths of the piston (front to back) just above the intake cutaway because this is the widest point of the piston. Check the maximum wear specs in your service manual. Check the piston for detonation marks in the crown, cracks in the skirt, or seizure marks. Look at the underside of the piston crown for a large black spot. The spot is burnt oil deposits that adhered to the piston because the piston crown temperature was too hot. This is an indication that the carb?s main jet needs to be richer.

Letter Designations on Cylinders and Pistons

The Japanese manufacturers use a letter designation system for plated cylinders. They intend for you to order replacement pistons based on the letter designation printed or stamped on the cylinder. This is the reason why they need this type of system. In mass production you can’t guaranty that all parts will be exactly the same size. The size variance is based on an acceptable level of quality. Tool bits become dull, temperatures of machine tools change through production runs, and machine operators have inconsistent performance. The Japanese manufacturers have between two to four different sized pistons and cylinders. Normally labeled A, B, C, and D. If they only had one size, the piston to cylinder wall clearance would vary between .001 to .006 inches. In the standard Japanese alpha labeling system, A denotes the smallest bore or piston size and every letter after that is slightly larger, usually in increments of .0015 inches. The danger is that if you try to put a D piston in an A cylinder the piston to cylinder wall clearance will be so tight that a seizure might occur.

Pro-X Oversize Piston Kits

Pro-X is a marketing company that sells the surplus pistons from the Japanese company ART, which makes all the cast pistons for the Japanese motorcycle manufacturers. These pistons are the same quality as the OEM pistons, and they are available in sizes larger than the alpha pistons available from franchised dealers. Also the Pro-X pistons are usually priced lower than the OEM pistons. If the cylinder bore is slightly worn (up to .005 inches) with only a small area of bare aluminum exposed, you can install a Pro-X oversize piston. The Pro-X pistons are graded oversize in smaller increments than Wiseco pistons, but a wider range than the OEM pistons. For example, Wiseco sizes are .010 inches and Pro-X is .001 inches increments. Before attempting to order a Pro-X piston, you must measure the cylinders bore at the smallest point and allow .002 inches clearance between the piston and cylinder.

Measuring the Ring Gap

The best way to know if the rings are worn is to measure the ring end gap. Put the ring in the cylinder and use the piston to push it down about 1/2 inch from the top evenly spaced. Now use a feeler gauge to measure the width of the ring gap. Normally, the maximum gap is 0.018–0.025 inch.

Cylinder and Exhaust Valve Cleaning

Does your cylinder have burnt-on mud on the outside, heavy brown oil glazing on the cylinder bore, or gooey oil on the exhaust valves? If so, here is a tip for cleaning those parts without flammable cleaners. Go to the grocery store and get a can of aerosol oven cleaner. This stuff is great for cleaning the carbon from the exhaust valves without completely disassembling them. CAUTION: Oven cleaner attacks aluminum, so don?t leave it on the cylinder for more than 20 minutes. Oven cleaner can be used on both steel and plated bores.

The oven cleaner will help loosen the oil glazing on the cylinder walls. Then, you can use a Scotch-Brite pad to hone the cylinder walls in a crisscross pattern. Wear rubber gloves when you use oven cleaner and flush the cylinder afterwards with soapy water. This will neutralize the acid in the oven cleaner and break the molecular bond of the oil, so the debris can be rinsed away. Sleeved (especially Kawasaki cylinder bores) are vulnerable to corrosion after cleaning. Spray some penetrating oil on the cylinder bore to prevent it from rusting.

Caution: Certain types of cylinders corrode quickly after the cleaning process, so spray the bore area with penetrating oil to displace the water.

Honing the Cylinder Bore

Many people have emailed me with questions regarding honing cylinder bores. If you want to buy a hone to deglaze bores or polish off small scratches, then a ball-hone is the best choice. Ball hones are manufactured by Brush Research in Los Angeles, under the brand name Flex-Hone. These hones are available under different labels and they are most easily available from auto parts stores. Buy a size that is 10% smaller than the actual bore size. These hones are available in several different materials and grits but the profile that bests suits both steel and plated cylinders is aluminum oxide 240 grit. A ball hone cannot remove material from the cylinder bore, especially on the hard nickel plated bores. However a ball hone can polish down the peaks of the original hone scratches and increase the bearing ratio. In other words the piston will be touching a greater percentage of the bore. Sometimes that makes the piston wear quicker but if you have to ball hone the bore to remove scratches, it?s a compromise. The one type of hone that you should never use on a two-stroke cylinder is a spring-loaded finger hone. The sharp edges of the stone will snag the port edges and most likely damage the hone and the cylinder.

Top End Assembly

1. Install one of the circlips in the piston with the opening facing away in the 6 or 12 o’clock position.

2. Grease the cylinder-base alignment pins.

3. Set the exhaust valves in the closed position.

4. On cylinders with reed valves, leave the intake port open because you will need to reach in through the port to push the piston-ring ends back in place.

5. The best way to slip the piston into the bottom of the cylinder is to rotate the rings toward one side of the locating pins and squeeze the rings with your middle finger and thumb. That will leave your other hand free to position the cylinder.

6. There are two methods used to assemble to top end. The first method is to attach the piston to the connecting rod and lower the cylinder on to the piston assembly. The second method is to install the piston assembly into the cylinder and lower the cylinder and piston on to the connecting rod. The second method is easier but involves pinning the piston and installing one circlip with a minimum amount of free space.

7. Take care to align the exhaust valve control mechanism as the cylinder is bolted to the crankcases.

Gasket Hygiene

The oven cleaner you used to clean the cylinders will help loosen the old gasket material so you can remove it. Carefully scrape the gasket off with a gasket scraper. Never use a flat screwdriver to remove the old gaskets because the aluminum surfaces of the head, cylinder, and crankcases are easily gouged. If these surfaces are gouged on your engine, they should be draw-filed flat to prevent air or coolant leaks.

Never reuse paper gaskets; always replace them with new gaskets, and spray sealer on the paper gaskets, so they will seal better and will be easier to remove the next time. The new-style steel gaskets can be cleaned and reused a few times, but you?ll need to spray the gasket with a sealer such as Permatex Spray-A-Gasket or copper-coat.

Keep a Logbook

Keep a logbook that tracks the number of riding days and the periodic maintenance. From reviewing the log, you will learn how often you need to service the top end if you record the measurements of the ring gap and the piston diameter. A logbook also gives you greater leverage when you try to sell your used bike for a premium price.

Big Bore Kits

One of the best ways to increase horsepower is to increase displacement by overboring the cylinder. This can be ideal for play or Vet Class riders, where the increased displacement won?t be illegal for your race class. When done right, a big bore kit can give you more power everywhere rather than an increase in only the top or the bottom of the powerband. Such increases are typically more usable and give you more power where you need it.

Piston manufacturers such as Wiseco make oversize piston kits for popular model bikes. These kits boost the displacement of the cylinder to the limit of a racing class or to a larger displacement class, for example: 80cc to 100cc, 125cc to 145cc, 250cc to 265cc or 300cc, and 495cc to 550cc.

The AMA has a limit of overboring any cylinder used in amateur modified classes. The limit is 2 millimeters. Wiseco makes a line of Pro-Lite pistons for this purpose. Normally no head modifications are needed, but cylinders with exhaust valves that operate close to the cylinder bore will need to be trimmed for clearance. Cylinders that use steel head gaskets will require oversize gaskets. Cometic makes 2 millimeter oversize and big bore gasket kits. The process of overboring and electro-plating a cylinder can be a cost effective way to save a cylinder that suffered a top end failure and scored the cylinder wall.

Riders competing in the AMA veteran class can ride a bike with any displacement. Riders competing in hare scrambles and enduro can race the 200cc class with a 125 converted to any displacement. AMA motocross and enduro racers can make the 250cc bikes legal for open class by increasing the displacement a minimum of 15 percent (to 286cc). Wiseco makes 74-millimeter piston kits to convert the popular 250s to 300cc. Be careful if you decide to go with a big bore kit, though. If the overbore is not performed properly, though, it can result in the wrong kind of power or, at worst, a ruined cylinder. When you change the displacement of the cylinder, there are so many factors to consider, such as port time-area, compression ratio, exhaust valves, carb jetting, silencer, and ignition timing. Here is an explanation of what you need to do when planning to overbore a cylinder.

Also, you should at least consult with an expert before tackling a big bore kit. To get the most from an overbored engine, you need to make sure the carburetion, exhaust, porting, and timing are all adjusted to suit the larger bore.

Port-Time Area

The term port-time area refers to the size and flow range of the intake and exhaust ports, relative to rpm. The ports enter the cylinder bore at angles. When the cylinder is over-bored the transfer ports become lower and wider. The same thing happens to the exhaust port. This effectively retards the port timing and reduces the total degrees of duration. When the displacement of the engine increases, so does the demand for more port-time-area.

If you just overbored and plated a cylinder, it would have much more low-end power than stock but the top-end power would suffer. Normally tuners have to adjust the ports to suit the demands of the larger engine displacement. The proper dimensions for the ports can be calculated using a computer program from Two-Stroke Racing (TSR) www.tsrsoftware.com The program “PORTTIME” enables tuners with limited math skills to run strings of formulas for determining the optimum dimensions of the ports. Generally speaking, if the ports in the overbored cylinder were raised to the same heights as the stock cylinder, that would make the port timing sufficient to run with stock or aftermarket exhaust systems.

Cylinder Head

After overboring the cylinder, the head?s dimensions must be changed to suit the larger piston. First, the head?s bore must be enlarged to the finished bore size. Then, the squish-band deck height must be set to the proper installed squish clearance. The larger bore size will increase the squish turbulence, so the head?s squish band may have to be narrowed. The volume of the head must be increased to suit the change in cylinder displacement. Otherwise, the engine will run flat at high rpm or ping in the midrange from detonation.

Exhaust Valves

When the bore size is increased, the exhaust valve-to-piston clearance must be checked and adjusted. This pertains to the types of exhaust valves that operate within close proximity of the piston. If the exhaust valves aren?t modified, the piston could strike the valves and cause serious engine damage. The normal clearance between the exhaust valves and the piston should be at least .030 inches or .75 millimeters

Carburetor

The larger the ratio between the piston?s diameter and the carb?s size, the higher the intake velocity. Overbored cylinders produce higher intake velocity which draws more fuel through the carb. Of course a larger engine will need more fuel. Normally when you overbore an engine 15-20%, the slow jet will need to be richened and the main jet will need to be leaned. Start with the stock jetting and make adjustments after you ride the bike.

Ignition Timing

The ignition timing has a minimal affect on the poweband. Retarding the timing has the affect of reducing the hit of the powerband in the midrange and extending the top end over rev. “Overrev” is a slang term that describes the useable length of the powerband at high rpm.

The scientific reason for the shift of the powerband to extremely high rpm, is because the temperature in the pipe increases with the retarded timing, and that enables the pipe?s tuned length to be more synchronous with the piston speed and port timing of the cylinder.

Advancing the timing has the affect of increasing the midrange hit of the powerband, but makes the power flatten out at high rpm. The reason is that the relatively long spark lead time enables for a greater pressure rise in the cylinder before the piston reaches TDC. This produces more torque in the midrange but the high pressure contributes to pumping losses at extremly high rpm.

Pipe and Silencer

Because only the bore size is changed, you won?t need a longer pipe—only one with a larger center section. FMF?s line of Fatty pipes work great on engines that have been overbored.

Head Gasket

The head gasket will need to have the bore diameter increased to the dimension of the new piston. If the head gasket overlaps into the cylinder bore more than one millimeters on each side, it could contact the piston or be susceptible to pressure blowouts.

10 TIPS FOR REBUILDING A TWO-STROKE TOP END

1) Before you disassemble your engine, power-wash the engine and the rest of the vehicle. That will reduce the risk of dirt and debris falling into the engine. Once you remove the cylinder, stuff a clean rag down into the crankcases.

2) The cylinder and head use alignment pins to hold them straight in position from the crankcases on up. The pins make it difficult to remove the cylinder from the cases and the head from the cylinder. Sometimes the steel alignment pins corrode into the aluminum engine components. Try spraying penetrating-oil down the mounting studs before attempting to remove the cylinder and head. Never use a flat-blade screwdriver, chisel, or metal hammer to remove the cylinder. Instead use this technique; buy a lead-shot plastic mallet, swing it at a 45-degree angle upwards against the sides of the cylinder. Alternate from left to right, hitting the sides of the cylinder to separate it from the cases evenly. Clean the steel alignment pins with steel wool and penetrating-oil. Examine the pins closely. If they are deformed in shape, they won?t allow the engine parts to bolt together tightly. This can cause a dangerous air leak or a coolant leak. The pins are cheap at about $2 each. Replace them if they?re rusty or deformed.

3) Never re-use old gaskets. Remove them with a razor blade or gasket scraper. Don?t use a drill-driven steel wool type pad to remove old gaskets because they can remove aluminum from the cylinder and head. That will cause a gasket to leak.

4) Always check the ring end gap on a new ring by placing it in the cylinder between the head gasket surface and the exhaust port. The gap should be between .012 to .024 inches.

5) Always install the circlips with the opening facing straight up or down, that way inertia will hold it tight into the clip groove. Place one clip in the groove before installing the piston on the connecting rod. Its easier to install a clip with the piston in your hand rather than on the rod. There also less chance that you?ll drop the circlip in the crankcases.

5) Always install the rings on the piston with the markings facing up. Coat the rings with pre-mix oil so they can slide in the groove when trying to install the piston in the cylinder.

6) Always install the piston on the connecting rod with the arrow on the piston crown facing towards the exhaust port.

7) The traditional way to assemble the top end is to install the piston assembly on the connecting rod, compress the rings, and slide the cylinder over the piston. That can be difficult with larger bore cylinders, or if you?re working by yourself. Try this method instead. Install one circlip in the piston, install the piston into the cylinder with the pin hole exposed, install the piston pin through one side of the piston, position the cylinder over the connecting rod and push the piston pin through until it bottoms against the circlip, install the other circlip. It only takes two hands to install the top end using this manor and there is less chance that you?ll damage the rings by twisting the cylinder upon installation.

On cylinders with reed valves and large oval intake ports, take care when installing the piston assembly in the cylinder because the rings are likely to squeeze out of the ring grooves. Use a flat-blade screwdriver to gently push the rings back in the grooves so the piston assembly can pass by the intake port.

9) For steel head gaskets, place the round side of the “bump” facing up. Don?t use liquid gasket sealer; use aerosol spray adhesive types instead. For hybrid fiber/steel ring head gaskets, place the wide side of the steel rings facing down.

10) When you initially start the engine after a rebuild, manipulate the choke to keep the engine rpm relatively low. Once the engine is warm enough to take it off choke, drive the vehicle around on flat hard ground. Keep it under 2/3 throttle for the first 30 minutes. Two common myths for proper engine break-in are; 1) Set the engine at a fast idle, stationary on a stand. 2) Add extra pre-mix oil to the fuel. When the engine is on a stand it doesn?t have any air passing through the radiator and it is in danger of running too hot. When you add extra oil to the fuel you are effectively leaning the carb jetting. This can make the engine run hotter and seize.

Top End FAQs

Thin Sleeve Causing Seizures

Question: My 1987 CR125 has chronic piston seizure problems. The cylinder is bored one millimeters oversize. The lower end was rebuilt so I know it doesn?t have a crankcase air leak. What could the problem be?

Answer: The original cylinder for your model bike had a very thin steel sleeve. Honda only offers one oversize piston. When the sleeve is overbored too far, the sleeve cannot transfer out heat into the water jacket efficiently. The heat builds up over the exhaust port, and the piston melts. You have two repair options: buy a new cylinder or install a new thicker sleeve in the old cylinder. Wiseco offers thick sleeves and forged piston kits.

Honda CR250 1988–91 HPP Problems

Question: My 1990 Honda CR250 is making me wacky. I tried to check the exhaust valve system, and I don?t think it works properly. I removed the left-side valve cover from the cylinder, revved the engine and the valves hardly moved. They don?t open fully when the engine is revved, and they don?t close completely either. What is the most common cause of this problem and how can I fix it myself?

Answer: The problem is that the HPP mechanism isn?t fully engaged, and the valves are just moving from the exhaust-gas pressure. The most common problem with the 1988–91 CR250 HPP systems, is the improper engagement of the governor control and the spindle rod that actuates the HPP valves. The following procedure may cure the problem. Remove the top right valve cover on the cylinder and the round-slotted access cover located under the water pump on the right side engine cover. Insert an 8mm T-handle through the access hole and onto the detent bolt that locks the governor control to the cam spindle, and turn the bolt 1/4 turn counterclockwise. Now, the bolt has disengaged the HPP system. Insert a straight-blade screwdriver into the slot in the top of the right-side pinion shaft (from the top right side of cylinder). Turn the pinion shaft counterclockwise 1/8 turn, and then turn the detent bolt (located under the right-side engine cover) 1/4 turn clockwise. It is important to release the spring tension from the pinion shafts in the cylinder to engage the detent bolt. This procedure also enables the HPP mechanism to be engaged without any chance of damage occurring to the fragile cam spindle.

Top-End Big Bore

Question: I have an old cylinder for my 250. The bore was ruined when the head gasket leaked, and there is severe erosion on the top edge of the cylinder. I read your article on top-end rebuilding and had an idea and a related question. I compete in amateur enduro events and the rules state that the displacement of bikes competing in the open class must be a minimum of 251cc. My question is, can I salvage this old junk cylinder by overboring the cylinder to fit a Wiseco piston kit and have the bore re-plated? If yes, will my bike be legal for the open class?

Answer: There are a number of companies offering cylinder repair services and replating. The way to fix the erosion problem is to heli-arc weld aluminum over the erosion and then re-face and bore the cylinder. WISECO and L.A. Sleeve make oversize piston kits and gaskets for most Japanese dirt bikes. The common overbore displacement sizes for 250s are 265, 285, and 310cc. After the cylinder is re-plated, the exhaust valves and the cylinder head must be matched to the larger bore size. This involves special metal machining and should be trusted to a qualified tuner or machinist. This type of mod will enable you to race your 250 in the open class.

Kawasaki Air/Oil Leaks

Question: My son and I are just getting started in dirt-biking. Over the winter I bought him a 1989 KX80 as a basket case. We are learning about dirt bike repairs by rebuilding this bike. It?s a lot like model building, only the parts are old and greasy! We inspected the crankcases and noticed that there was some oil leaking from the three oval-shaped plugs that are spaced an equal distance around the main bearings. How can we repair this problem without buying new crankcases?

Answer: Every Kawasaki dealer?s service department has a Team Green book with tips on how to repair common problems. Ask your dealer?s service manager for a copy of the Team Green bulletin. It has photos and drawings of how to apply the epoxy over the crankcase plugs.

Top-End Seized After Rebuild

Question: I trail ride a 1989 YZ250. Last winter, I rebuilt the top end after reading your article in Dirt Rider. The bore was so worn that I had to skip to a one millimeter-oversize piston kit, just so the bore job would clean up a severely worn spot below the intake port. After I rebuilt the top end, I cycled the engine by letting it idle for three 15-minute sessions with adequate cool-down periods in between. When I first rode the bike, I heard some detonation noises but didn?t think it was a serious problem, until it seized. What could be wrong?

Answer: Your problem is simple. When a cylinder is overbored, the displacement is increased and that boosts the compression ratio. Whenever a cylinder is overbored more than 0.010 inches or 0.25mms the cylinder-head diameter must be enlarged to the new bore size. Otherwise, the piston could contact the head or the edge of the head surface that extends into the bore could cause a hot-spot and pre-ignition. Also, the cylinder head?s squish band must be narrowed by enlarging the combustion-chamber bowl. This also serves to increase the head?s volume, thereby lowering the compression ratio. This work must be performed on a lathe by a qualified tuner or machinist. Average cost of this service is $50

Base Gasket Seeping

Question: I recently rebuilt the top end on my 1991 CR250. I was being as careful as I could be while taking the cylinder off, but the dowels were fused in pretty good and I had to pry it. Needless to say, I gouged the case a bit. I smoothed it out with sandpaper and reassembled the engine. The bike runs great, but a little oil seeps out of the cylinder-to-case mating surface. I assume this is transmission oil? Would it be OK to use something like a thin layer of Permatex Blue or Yamabond here? Would this make it even more difficult to remove the cylinder in the future? Should I just let it alone? The best price I could find on a new left side case was $215 and I?m sure it would be a lot of work and a lot of replacing gaskets along the way. Am I out of luck?

Answer: Air leaks can be very dangerous because the engine could rev independent of the throttle. An inexpensive way to fix your bike?s problem is to draw-file the cylinder base and the crankcases. Then apply a thin coating of Yamabond or any other brand of non-drying sealer to both sides of the base gasket. The best technique for removing cylinders is to tap up on each side of the cylinder with a lead-shot plastic mallet. Remember to put a dab of grease on the cylinder-base dowel pins.

Frequency of Top-End Rebuilding?

Question: I have a 1990 RT180, and I don?t think the rings or piston have been replaced. I don?t know if the top end has ever been rebuilt because I bought the bike used. How long do piston and rings usually last on a two-stroke engine like mine? How often should the piston and rings be replaced, and should I replace them now?

Answer: Replace the piston and rings before they wear out. The time scale varies between models, usage, and preventive maintenance. The only way to determine the condition of your bike?s top end is to disassemble the top end and measure the piston diameter and the ring end gap. Compare the measurement to the maximum wear specs published in the service manual.

Two-Stroke Exhaust Valves

Three words sum up exhaust valve maintenance: spoogey, gooey, and grungy. If two-stroke exhaust valves didn?t have such a dramatic effect on the engine?s powerband, I?m sure mechanics would remove them and beat them bits with a hammer in frustration because there is little information given by the manufacturers on how to diagnose and repair the exhaust valve systems on well-used dirt bikes. This section is a guide to the characteristic mechanical problems that occur to the exhaust valve systems of dirt bikes. Plus we?ll give you some tips on how to re-time exhaust valve systems.

How Exhaust Valves Work

An exhaust valve system is designed to increase the engine?s low-end and midrange power. There are three different designs of exhaust valve systems. The first-generation design uses a variable-volume chamber mounted to the head pipe to change the tuned length of the head pipe. A butterfly valve is used to separate the surge chamber and the head pipe. At low rpm, the valve is open to allow the pressure waves in the pipe to travel into the surge chamber and effectively lengthen the pipe and reduce the pressure wave?s magnitude when it returns to the exhaust port. These systems were primitive and not very effective on 125cc dirt bikes. Honda and Suzuki used this type of exhaust valve system in the mid to late 1980s.

The second-generation design features valves that control the effective stroke and the time-area of the exhaust port. These valves are fitted to the sub-exhaust ports and the main exhaust port. The main exhaust-port valves operate within close proximity to the piston to control the effective stroke of the engine. The effective stroke is defined as the distance from TDC to when the exhaust port opens. At low rpm, the engine needs a long effective stroke, which results in a high compression ratio. At high rpm, the engine needs a shorter effective stroke, longer exhaust duration, greater time-area, and a lower compression ratio. Yamaha used this system starting in 1982 on the YZ250. Honda?s HPP system is similar and was used on the 1986–91 CR250 and 1990 to current-model CR125.

The third generation of exhaust valve systems attempts to change the exhaust-port velocity, effective stroke, exhaust-gas temperature, and the pressure of the compression wave. Yamaha and Suzuki started using these systems on their 125s in 1995. Both companies employed a venting system to the outside atmosphere. This is very complex because they are attempting to affect the temperature and pressure of the returning compression wave to synchronize it with the piston speed. The exhaust-gas velocity and the effective stroke are controlled by two oval wedge valves that enter the exhaust port at a 45-degree angle. The wedge valves partially block the exhaust port, thereby boosting the gas velocity. Kawasaki?s KIPS system uses wedge valves in the main exhaust port to control the effective stroke, drum valves in the sub-exhaust ports to control the time-area, and a surge chamber to absorb the excess compression-wave pressure at low rpm.

The exhaust valves are opened and closed by a centrifugal governor mechanism. The governor is mounted under the right side cover and is gear-driven by the crankshaft. As the engine rpm increases, the governor spins, thereby increasing the angular momentum of the four steel balls encased in the governor. The steel balls fit into an angled ramp-and-cup arrangement. A spring is used to provide tension on the steel balls. When the momentum of the steel balls overcomes the spring?s tension, and the balls force their way up the angled ramp. A spool attached to the ramp, enabling it to change its linear position with changes in rpm, and the spool is attached to a linkage system that operates the exhaust valves in the cylinder. Factory race teams have different combinations of springs, ramps, and balls to tune the exhaust valve operation and enhance the powerband.

Exhaust Valve Tips and Tuning

Although exhaust valves use the same essential principles, the implementation is different with each manufacturer. Also, each type has its own flaws and fixes. The list below gives you tips on how to install and service the most common exhaust valves, as well as some tuning tips

Honda HPP

Honda?s HPP system started as a butterfly operated canister mounted between the cylinder and pipe. It served to control the volume and length of the exhaust pipe. It had little effect on the power and most aftermarket pipes eliminated the canister. The butterfly was prone to carbon seizure and required frequent maintenance. The next generation HPP was used on the 1986-91 CR250. This system featured two sliding valves that operated within close proximity of the piston and effectively varied the exhaust port time-area in accordance with rpm. The square valves moved horizontally through a valve guide. The system was plagued with a mixture of design problems and misinformation on how to service and re-time this complicated exhaust valve arrangement. This section lists some common problems and some tips for timing the system, installing the cylinder, and engaging the HPP mechanism.

Common HPP Problems

Two main problems plague the HPP system: carbon fouling and rack-and-cam-spindle damage. The square shape of the valves contributes to the accumulation of carbon on one corner of the valve guide (stationary part), in the corner of the guide that is directly in the exhaust gas stream and this causes the valve to become carbon seized. Chamfering the corresponding edge (one-millimeter) of the valve will eliminate this problem. The rack and cam spindles are easily damaged when the cylinder is installed incorrectly, or the HPP mechanism is engaged incorrectly. See the photos for examples of damaged rack and cam spindle parts.

HPP Timing Procedure

Use the following procedure to time the HPP system:

1. Install the HPP valves and levers and tighten the pivot nuts. Place the washer on the stud first, then the lever (marked left and right), and then the flanged center bushing with the flange side facing up.

2. Turn the cylinder upside down. To position the rack correctly, slide it to the left until it stops; then move it right 2mm. Rotate the rack so the square notch faces you. Now the rack is in the correct position so you can install the pinion shafts. Carefully turn the cylinder right side up without changing the position of the rack.

3. Close the valves and install the left pinion shaft with the screwdriver slot facing the one o?clock position. Install the right pinion shaft with the screwdriver slot facing the eleven o?clock position (see photo for correct positions). A simple way to determine if the pinions are mis-timed to the rack is to look at the screwdriver slots. The wrong position is with both slots facing twelve o?clock.

Installing the Cylinder and Engaging the HPP Drive

After timing the HPP mechanism, the cylinder is ready to be installed on the crankcases. Here are some tips for installing the cylinder and engaging the HPP drive mechanism:

1. Make sure the reed valve is removed from the cylinder. CR250s have such large intake ports that the rings tend to slip out of the ring grooves during installation of the cylinder. This takes the spring pressure off the cam spindle. Now turn the engagement bolt 1/4 turn clockwise. You should feel it positively lock into a groove and stop. Remember that the HPP engagement bolt is a spring-loaded detent not a threaded bolt. Slide the cylinder down onto the piston and rings, use a screwdriver to push the rings back in the grooves until the rings clear the intake port.

2. The HPP mechanism should be engaged while the cylinder is being installed, just to keep the cam spindle in position. The cylinder will stop about 3mm from the crankcases because the cam spindle and the rack are misaligned. Now disengage the HPP mechanism by turning the engage bolt 1/4 turn counter-clockwise. Grasp the right-side valve lever and wiggle it; the cylinder should then drop evenly onto the crankcases.

3. Bolt the cylinder down tight. The best way to engage the HPP mechanism is to insert a screwdriver in the right-side pinion shaft and turn it counterclockwise. Now turn the engagement bolt clockwise. You should feel the engagement bolt lock positively in position. If you try to rotate it too far, you will bend the cam spindle and the system won?t work at all, so don?t be a hammer-head! The best way to check the HPP system is to remove the left-side valve cover from the cylinder, start the engine and warm it up, then rev the engine. The valves should be fully closed at idle and fully open when the engine is revved.

In 1992 Honda introduced the HPP system currently used on the CR250. This system features a center valve for the main exhaust port and two rotating drum valves to control the flow of the sub exhaust ports. This system also features a return of the old resonator as used on the mid-eighties model. The resonator improves the throttle response and mellows the powerband at low rpm. A thin rod links the valves together and the whole system is mostly self-scraping to prevent carbon build-up. The inside of the center valve has an elongated passage where the tie rod travels. This elongated passage is prone to carbon build-up over time (1-2 years). The carbon limits the range of movement in the valves. The carbon is easily removed by using a small diameter rat-tail file. The sides of the center valve and the drum valves interface, and that area is prone to carbon build-up too. A wire brush or file is an effective tool in cleaning the exhaust valves. Here is a simple way to check the operation of this system. On the left side of the cylinder there is a 17mm cap bolt that exposes a straight line mark in the left drum valve. There is a corresponding mark on the cylinder. The “L” mark denotes the low speed position of the valve and the “H” denotes the high-speed position. To check the HPP, start the engine. At idle the valve should align with the “L” mark. Then rev the engine, the valve should align with the “H” mark. If the angle of the mark on the valve is slightly off, then the valve probably needs to be de-carboned. This system is very easy to disassemble and can only fit together one obvious way so we won’t waste space on that procedure. There is some aftermarket parts to adjust the performance of this system for different types of dirt biking. Pro-Racing in England makes a spacer for the right side valve cover. It serves to add volume and length to the resonator part of the system. This is especially suited for enduro riding where a smooth transition to the mid-range is important for better traction. ESR (Eddie Sanders Racing) in California makes a replacement HPP system that holds the valves wide-open. The center exhaust valve is thinner which enables tuners to raise the exhaust port. The ESR system is primarily used for dirt track or kart applications where low-end power is of no consequence.

Whenever the cylinder is installed on the bottom end after top end rebuilding, the valves need to be put in the closed position. Otherwise the HPP cam spindle that connects the actuator in the cases to the cylinder will get damaged when you tighten down the cylinder. That will also make the valves inoperable. Always check the HPP valve operation after you assemble the top end by using the inspection cap on the left side of the cylinder.

The CR125 HPP system was redesigned in 1990. Honda chose to use a system similar to the 1986-91 CR250, featuring horizontally sliding valves. This system was plagued with problems over the years. The valves are prone to carbon seizure because the critical square edges face the exhaust stream. If the clips that fit on the ends of the valves vibrate off or if the valve wears too much then the valve can tilt on an angle and strike the piston. Another common related problem happen when tuners widen the exhaust port during porting and neglect to grind the valves at the outer corners for piston clearance. There again the piston strikes the valves because they protrude into the bore. In 1998 Honda made a modification to the valves, they added an L-shaped rib that prevented the valves from angling in and contacting the piston. The other problem of clearance between the top of the valve and the guide was eliminated so the new style valves provide more low-end power. These valve and guide sets from the 1998-99 models fit the CR125 models back to 1990.

In 2000 Honda redesigned the CR125 engine and adapted the exhaust valve system used on the RS250 roadracer. Honda also used this system on several dual sport and street bikes sold in Asia and Europe. The new system is so simple and effective. It is a wedge-shaped valve that pivots at one end, similar to the CR250. The valve is much thicker and can vary the exhaust port’s effective stroke, time-area, and duration over a wider rpm range. It?s a self-scraping set up so maintenance should be greatly reduced over previous models.

Kawasaki KIPS

Kawasaki?s KIPS exhaust valve system has gone through a steady refinement of design. Kawasaki uses a different system to suit the needs of the different model bikes. The earliest KIPS design used two drum shaped valves to control the flow of the sub exhaust ports. Opening the ports gave the exhaust port more time-area. The main exhaust port was relatively small with modest timing and duration. A rack and pinion set up controlled the drum valves, opening them at about 6,000 rpm. Kawasaki uses the rack and pinion design in all their KIPS systems except the 1998 and later KX80-125cc models. The 1992 KX125 and KDX used the next generation KIPS which featured a center wedge valve with two side drum valves engaged to a rack-and-gear actuating system. This system was very complicated with all its moving parts. The top and bottom racks had to be synchronized through the left drum valve, which has two drive gears molded in it. The drum valves are made of aluminum. When the drum valve becomes carbon seized, the steel teeth on the rack shear off the aluminum teeth on the drum valve, rendering the drum valve inoperable. Check the condition on the gear teeth every time you do a top-end service, because if one gear fails the whole system runs out of sync. . On the late model 80-125cc KXs, the KIPS is relatively simple relying on a wedge valve and flapper. This system is self-scraping so it requires little maintenance. In the first year of operation (1998) the KIPS system was plagued with failures like the pin breaking on the flapper, the valve receding into the cylinder and contacting the piston, and over-extension of the valve causing cock and jam. Pro-Circuit made an aftermarket valve cover with a full stop that prevented over-extension and in 1999 Kawasaki changed the wedge valve and flapper design for more rigidity and that solved all the reliability problems.

The drum valves on the 1988- 92 KX250 and 1990-2000 KX500 are also aluminum but have a hard-anodized coating that resists wear. However, the drum valves eventually wear at the drive channels for the center wedge valve, and the sloppy fit between the wedge and drum valves prevents the center valve from fully opening. That is why these bikes get noticeably slower as they get older. There is no preventative cure or aftermarket part. You just need to replace the drum valves when the drive channels wear out. The 1993 KX250 was the first year for the KIPS system used through present day models. The system uses a single wedge and flapper valve for the main exhaust port and two drum shaped valves for the sub exhaust ports. The valves are all linked together with two racks and pinions on the right drum valve and a steel gear on the upper rack linking the wedge valve. A left-hand-thread nut retains the gear to the rod that actuates the wedge valve. Check the nut periodically, if the nut loosens, the wedge valves become inoperable. The KX250 KIPS also features two large cavities to allow for dissipation of the compression wave that travels back up the exhaust pipe at low to mid rpm. It’s important that the two valve covers on the cylinder be sealed with gaskets and it is normal for large amounts of black sludge to accumulate under those valve covers. It takes years for the sludge to accumulate to the point of adversely effecting performance. The only way to clean out the sludge is to have the cylinder hot-tank cleaned at an automotive rebuilding store. The 1993–2000 KX250 wedge valve tends to form burrs at the outer edges that face the piston. These burrs prevent the wedge valve from opening fully, and the thin flap that comprises the exhaust-port roof hangs out into the exhaust-gas stream, producing a shock wave that closes off the exhaust port. File the burrs smooth and check the wedge valve through the full range of movement. The valve pocket in the cylinder gets worn too. Aftermarket cylinder rebuilders like US Chrome apply a hard coating to that area to reduce wear or build-up material that has worn down from the moving wedge valve. Another characteristic problem of the KX250 KIPS is broken governor levers. The lever that transmits the movement from the centrifugal governor to the right-side case lever tends to break in half. This piece is located under the right side cover. If your KX250 suddenly loses top end power, its probably due to the actuating lever breakage or the carbon-seizure of the KIPS valves.

1988–92 KX250 and 1990-2000 KX500 KIPS Timing Procedure

The explanation of this procedure, written in the Kawasaki service manual, is confusing. It requires you to time the upper and lower racks at the same instant. My method of timing the exhaust valves is composed of simple steps that enable you to check your work as you go. The 1988–92 KX250 and KX500 use the drive-channel system to actuate the center valve. Here is the best way to time the KIPS on these models.

1. Set the cylinder upside down on a bench.

2. Install the center valve but don?t bolt it in!

3. Install the side drum valves and align the drive channels on the drum valves with the center valve, but don?t bolt it in!

4. Install the side drums valves and align the drive channels on the drum valves with the engagement pins on the center valve.

5. Lift up the drum valves so the bottoms of the gears are flush with the cylinder base. Take care not to disengage the center valve.

6. Slide in the rack from either side of the cylinder. Position the rack by installing the seal pack and pulling the rack out until it bottoms against the seal pack. This is the full-open position.

7. Drop the drum valves onto the rack so the valves are in the full-open position. Don?t pay attention to alignment dots or marks on the valve or rack just remember that the valves should be open when the rack is pulled out and closed when the rack is pushed in.

1992–97 KX125 and 1993-2000 KX250 KIPS Timing Procedure

The system used on the KX125 and KX250 uses both wedge and drum valves with racks. This is the best exhaust valve system for performance but the most difficult to maintain. Here are some tips for re-timing this KIPS system.

1. Install the wedge valves in the cylinder and the actuating rod and lever. Squirt some pre-mix oil on the parts.

2. Pull the wedge valve into the full-open position, place the gear on the end of the rod, and rotate the gear counterclockwise until the rack butts against the stop plate. Thread the nut on the rod and tighten it counterclockwise because it is a left-hand-thread nut.

3. Place the drum valves into their respective cavities until the top of the gears are level with the cylinder base. Now push the lower rack into place and bolt the seal pack on the rack into the cylinder.

4. Pull the rack out until it stops and push it in one millimeter; now it is in the correct position to install the drum valve. Before you push down the drum valves, make sure the wedge valve and drum valves are in the full-open position.

5. Push down the drum valve with the two gears first because it must engage the upper rack and lower rack simultaneously. Take care and be patient. You may have to wiggle the wedge valve yoke to get everything to fall into place. Never hammer the drum valves! Then push down the right drum valve and install the idler gear. Now install the bushings and check the system. The valves will bind and stick if you try to move the valves without the bushings installed, or if the cylinder is facing upside-down. Test the KIPS in this way, pull the rack outward until it stops, look through the exhaust port from the pipe side. The valves should be in the full open position. On cylinders where the base has been turned down more than .010 inches, the drum valve bushings will also need to be turned down to prevent the valves from binding when the cylinder is tightened.

Suzuki ATEV

Suzuki first used exhaust valves in 1985, using a drum valve that uncovered a cavity in the head or cylinder to add volume and length to the exhaust pipe, strictly at low rpm. In 1987 they employed a system that featured two large valves that had multiple functions. This system was used on the 1989-2000 RM80, 1987-2000 RM125, 1987-95 RM250. The wedge shaped valves was positioned at about a 45-degree angle over the exhaust port. The ATEV system is designed to regulate the effective stroke, exhaust-gas velocity through the exhaust port, and on 1995 and later models it controls the exhaust gas temperature. The ATEV system is self-cleaning in that the valves are scraped of carbon every time they move. Some of the early-model RMs suffered from broken exhaust valves when the stem would detach from the cylindrical wedge. That problem was cured in 1991 when the radius between the stem and valve was increased. The two common problems that occur with the ATEV are caused by the two following errors in assembling the system:

1. Too much preload on the spring. On the left side of the cylinder is a dial that controls the spring preload for the exhaust valve system. The preload doesn?t have that great of an affect on the engine?s powerband, but too much preload will prevent the valves from opening, which causes a lack of top-end power.

2. Crisscrossed spring. A centering spring on the right side of the cylinder, located on the rod, actuates the valves. This spring is commonly installed wrong. The spring tabs should be parallel when coupled to the lever and rod. If the spring tabs are crisscrossed, the valve travel will be limited and won?t open fully.

In 1996 Suzuki redesigned the RM250 engine, going back to a design reminiscent of the 1987 model RM250. For this model Suzuki modified Honda’s HPP design used on the late model CR250. However a problem plagued this system. Instead of pivoting the center valve, Suzuki chose to slide it in a passageway of the cylinder. The added mechanical friction made the system prone to binding in one position, half-open. This causes the engine to run flat. Another problem was the shape of the valve. The leading edge that faced the piston was too square and sharp. Even when the valve was in the full open position it caused a shock wave that impeded the outgoing exhaust flow. Grinding the edge smooth reduced the low-end power but helped improve top end. In 1997 Suzuki redesigned the center valve, choosing steel as a material and splitting the valve into two sections, a major and minor valve. They also added a two-stage spring system. With some simple grinding to match the valve to the exhaust port when fully open, this set up was a winner! Suzuki chose to redesign the system in 1998-2000 to the 1997 design. The thought was that the steel valve damaged the valve pocket in the cylinder. Although simply extending the nickel silicon carbide bore material into the valve pocket would’ve solved this problem.

Yamaha POWERVALVE

Yamaha was the first motorcycle manufacturer to adapt exhaust valves to two-stroke motorcycle engines. Yamaha?s simple design of a cylindrical valve that rotates 1/4 turn to vary the height of the exhaust port requires little maintenance. This system was used on the YZ250 from 1982-98, and on the YZ125 from 1983-93. Occasionally, you have to replace the seals and O-rings to prevent exhaust oil from drooling out of the side if the cylinder. In 1989, Yamaha added a stop plate to limit the travel of the power valve, primarily so mechanics couldn?t install the valve in the wrong position. The stop plate is located on the left side of the cylinder. The valve has a small tab that bumps up against the stop plate to limit the fully open and closed position of the valve. This design enabled Yamaha to position the valve closer to the piston to make it more effective at varying the exhaust-port timing. Unfortunately, the soft-aluminum tab on the valve gets worn, allowing the valve to rotate farther in the fully closed position. Eventually, (after about three years? use) the tab wears enough so the valve strikes the piston, causing damage to the piston. Yamaha?s exhaust valve is cheap to replace. I recommend replacing the valve when the tab wears more than 0.030 inch (0.7mm).

In 1994 Yamaha changed the engine design of the YZ125 and included the next generation of exhaust valves. This system used two oval-shaped wedge valves, positioned at a 45-degree angle over the exhaust port. This system was similar to the one employed by Suzuki. Yamaha experimented with resonator cavity volume, and vents for pressure bleed off and temperature control. Overall this is a very reliable system. Occasionally the pins that fit through the ends of the valve to interface with the actuator lever vibrate out causing the valve to strike the piston. Those pins are a press fit but you can add some Loctite Instant Adhesive to the pins for added protection. One problem that Yamaha is concerned with is high rpm valve flutter. They’ve added springs to the valves to control the flutter but future innovations could include a positive seal between the valve and the cylinders’ valve pocket.

In 1999 Yamaha redesigned the YZ250 engine and exhaust valve system. This model features a powervalve that marks a significant design change, from the company that pioneered the use of exhaust valves on two-stroke engines. Looking more like a Rube Goldberg device, the new powervalve has separate valves for the main (center) and sub-exhaust ports (sides). The whole assembly is controlled by one actuating rod, but the side valves open after the main exhaust valve. The side valves are controlled by two wedge-shaped ramps but you can bet that the factory teams are experimenting with them. The stop plate of the center valve tends to crack, allowing the valve to contact the piston. Look for cracks in the plate richt around the two retaining bolts.

Article content: Eric Gorr

Related posts:

1. Engine Build HPP Valve Honda CR250 – Part 7
2. Engine Build HPP Valve Honda CR250 – Part 9
3. Engine Build HPP Valve Honda CR250 – Part 8

If you are looking to freshen up your older YZ tank and shrouds, here’s how you do it.

Tools needed for the conversion:

Box cutter
Hacksaw
Hand files
Center Punch

Vice to hold it all
6.5mm Drill bit for the tank mounts
Step Drill bit capable of drilling a 16mm hole for the rubber bushings

Start by sliding the rubber tank mount up the frame and re attaching with a zip ty. The lower rubber mount had to be trimmed (Razor Blade/Box Cutter) on the side so the tank would slide over it snug. (picture shows the radiator cages I built).

Here are the brackets I made. They are made from 40×40 3mm Aluminium angle. The rubber bushings with washer tube inserts are the same used on the radiators. Extra holes in the middle of the bracket are just for visual.

Here are the dimensions for the mounting holes for the Tank. All measuring from the TOP leading edge downwards.

Use your 6.5mm drill bit to drill the holes.

Radiator fill side measurements. (Right)Measurements for the left side

For the lower shroud brackets you can do this (Taken from another build)  Drill 2 holes 25mm’s apart (center to center). I’d use a bit of smaller angle for extra strength so there is a spine, bend the bracket for the correct fit to your shroud.

This is how I did mine, I built Radiator cages with integrated lower shroud mounts for the newer guards. 6mm side thickness and crush tubes bolted front and rear. Shouldn’t be able to wreck those babies in a hurry!

Lower shroud mount

 Thanks to Ben Rowe for the information and pictures.

Related posts:

1. 2010 Honda CR250 ELSINORE
2. Engine Build HPP Valve Honda CR250 – Part 3
3. Cleaning and the Pressure Washer

The majority of off-road motorcycles have poor overall chassis setup.

 One of the major differences between a beginner’s bike and a factory rider’s bike is the experience & knowledge of the person performing the setup. Unfortunately many riders just ride their bikes with no thought of chassis setup, and only do the bare minimum required maintenance. This is unfortunate as not only will the bike’s performance and your enjoyment of the sport suffer, but having the proper setup offers enormous advantages in SAFETY!!

 There are a lot of handling problems associated with incorrect chassis setup. Having the proper spring rates, race sag, head bearing tension, tire pressures, and freshly blead forks, are the first steps in getting your bike to handle better. ALL of these steps must be done prior to performing suspension adjustments, and certainly before attempting revalving.

Race Sag/Static Sag:

 The most common chassis setup problem is incorrect race sag/static sag. The proper amount of race sag for a full sized Japanese motocross bike is 95-105mm (unless you have a track record using a different setting). The proper amount of free or static sag is 25-40mm. KTM’s can run as low as 110mm race/40mm static sag. Contrary to popular belief, 2mm difference in race sag makes a dramatic difference in overall handling!

 Step 1: Put bike on a stand with both wheels off the ground.

Step 2: Take a tape measure and measure the FULLY EXTENDED length of the rear suspension. This is done easiest by measuring from the rear axle to a spot on the rear fender that is directly above the rear axle. Put a small piece of tape on the fender and leave it there as a consistent reference point. DO NOT MEASURE FROM THE REAR AXLE TO THE SEAT BOLT, AS THIS USUALLY ISN’T EXACTLY VERTICAL. Write down this measurement.

Step 3: Put on all of your riding gear that you would normally race with. Take the bike off the stand. Have someone stand in front of the bike, holding it upright. Sit on the bike in the attack position, with your feet on the pegs.

Step 4: Have a friend pull down slightly on the rear of the bike, then SLOWLY let it come back up on its own. Measure the exact distance between the same reference points. Write down this measurement.

Step 5: Now have a friend lift up slightly on the rear of the bike, SLOWLY letting it settle. Measure between the reference points again. Write down this measurement. The average of these two measurements is the TRUE RACE SAG. If the two measurements are more than 5mm different, the rear suspension linkage assembly is sticking and must be serviced. If the race sag isn’t between the 95-105mm range, it can be adjusted by tightening or loosening the shock spring preload with the spring’s retainers.

Step 6: Stand beside the bike and hold the the bike straight up by the end of the handlebar. Have a friend slightly compress the rear suspension, then SLOWLY let it extend on its own. Measure between your reference points. Write down this measurement.

Step 7: Now slightly pull up on the rear of the bike, then SLOWLY let the rear end settle under its own weight. Once again, Write down this measurement. The average of these two numbers is the TRUE STATIC SAG. As with the race sag, if the two numbers are any more than 5mm different, your linkage needs service. If the free sag isn’t between 25-40mm, WITH THE CORRECT RACE SAG, you will need a different rear spring rate. If there isn’t enough static sag, your spring is too soft. If there is too much static sag, your spring is too stiff.

IF YOU DO NOT CHECK YOUR SAG USING THIS METHOD, YOUR CHASSIS SETUP & SAG WILL BE INCORRECT. THIS IS ABSOLUTELY THE BEST, MOST ACCURATE PROCEDURE FOR SETTING SAG HANDS DOWN.

ASV Sag Scale

Incorrect Motorcycle Tire Pressure:

This is an extremely important yet often overlooked chassis setup as 90% of riders do not set their motorcycle tire pressures properly! The majority of riders just set their tire pressures somewhere between 12-15 psi, and never really give it much thought. Too much motorcycle tire pressure will tend to make the suspension feel stiff, and too little will make the bike wallow and push.

There is only one method to deciding the proper motorcycle tire pressure and it is called RIM CLEAN. There must be up to a 4-5mm maximum clean strip on the outside of the rim where it contacts the tire. During use the tire rolls over the rim somewhat, keeping this part of the rim clean. ALL off-road motorcycle tires are designed to work this way, front and rear. “D” shaped rims will require less rim clean for obvious reasons.

The motorcycle tire pressure is adjusted so that the proper amount of rim clean is visible. The tire pressure will differ dramatically from tire to tire, tube to tube, bike to bike, rider to rider, and track to track. A soft carcass tire with a stock tube may require 16 psi in order to have the proper rim clean. A hard carcass tire with a heavy duty tube, may only require 6psi.

It is very easy to run the wrong tire pressure, but most people don’t know how to calibrate it. If you have the proper amount of rim clean, and your buddy has the exact same bike/tires/tubes but he has no rim clean, you theoretically have say 10% MORE TRACTION! Tire pressure (psi) is only a number, and that number is used to calibrate the rim clean.

Steering Head Bearing Tension:

Steering head bearing tension is another important chassis setup that is often overlooked. Place the bike on the stand, with BOTH wheels the same distance off the ground. Slowly turn the bars from side to side. There must NOT be any “crunchy” feeling spots throughout the sweep. If there is, the head bearings must be replaced. With the proper amount of bearing tension (and a well serviced bearing set), the wheel will stay 1″-2″ OFF OF CENTER BEFORE IT FALLS TO THE STOP. If the wheel won’t stay centered on its own, the bearings are too loose. If the wheel stays more than 2″ off of center, without falling to the stop, the head bearings are too tight. This is a general guideline. Most riders prefer it this way. There is absolutely no bad handling traits associated with the head bearing tension, if they are set using this procedure. Slightly loose bearings tend to make the bike want to head-shake more while braking hard, and it makes the bike feel sloppy. Slightly tight bearings tend to make the steering darty, especially on the face of a jump. Both increase arm-pump!

Front Forks Air Pressure Build-Up:

Many riders aren’t aware of the fact that your forks need to have the excess air blead from them for proper performance and chassis setup. Many riders that are aware of it do not perform the procedure properly. Start off by placing your bike on a stand, so that at least the front wheel is off the ground. Take a small screwdriver, and crack the bleeder screw loose from the top of each fork leg. DO NOT try and turn the damping adjuster. The bleeder screw is located to the side of the fork cap, NOT in the center. Leave this screw loose for a few seconds, until you hear no further air escaping, then re-install it. DO NOT DO THIS WITHOUT THE BIKE BEING ON THE STAND!!! (front wheel off the ground). It is best to do this before and after every ride as it will make the suspensions’ performance more consistent.

Fork Misalignment:

It is very important that your front axle not bind in the fork legs for proper chassis setup and performance. Most modern front axles accept a 19mm Allen socket. The proper method of installing the front wheel is to use a wrench to hold the nut, and turn the axle using the 19mm Allen socket. Once the axle has been tightened to the proper torque, the fork dropout pinch bolts can then be tightened. By spinning the axle in, you guarantee the axle is not bound up in the fork dropouts.

Rear Axle Position:

The rear axle block on most off-road motorcycles have over 20mm of adjustment. This is a very important area in proper chassis setup as depending upon where you place your rear wheel will alter the wheelbase, front/rear weight distribution, and cause a very different reaction on the performance of the rear shock. When the rear wheel is all the way forward, there is less leverage on the rear shock and will make the rear motocross suspension feel stiffer. This position will also reduce the overall wheelbase of the bike, and distribute less weight on the front tire and more weight on the rear. Mounting the rear wheel farther back will have the effect of softening the rear shock. This will lengthen the wheelbase and tend to distribute more weight on the front tire.

This artcle was found, if anyone knows the publisher let me know and I will add credit.

Related posts:

1. The Straight Dope: Which Handles Better, A Two-Smoke or a Thumper?
2. Artrax 125/250F Tire Combo Review
3. Sprockets and Gear Ratio’s – What you should know

What Is Final Drive Ratio: The final drive ratio can be calculated by dividing the number of teeth on the rear motorcycle sprockets by the number of teeth on the front or countershaft sprocket. For an off-road dirt bike the result will typically range from approximately 3.5 to 4.5. This number represents how many times the countershaft sprocket will turn in order to turn the rear motorcycle wheel one revolution. A higher number equates to more turns of the countershaft sprocket and is referred to as “Lower Gearing”. Conversely, a smaller number is referred to as “”Taller Gearing”.

Lower Gearing means the engine speed will be higher (Higher RPM’s), while Taller Gearing lets the engine work at Lower RPM’s at a given vehicle speed. 

Why Is The Final Drive Ratio Important? Let me fill you in on a dirty little secret of motocross racing….despite what many hop-up shops and dyno numbers are being advertised “It’s Not All About Maximum Power”!! Yes you heard that right…..motocross or any off-road motorcycle & atv racing is about creating “Usable Power For Given Rider Skill & Track Conditions”. It is possible that the engine with the greatest peak horsepower will not produce the best acceleration because the driving force will be deficient at all speeds except where peak power is produced. If maximum power was all we were concerned about then motocross racing would be nothing more than a drag-race. It is acceleration that usually wins races, and this is important!! Do yourself a favor, before spending thousands of dollars on high performance engine “hop-ups” first get your motorcycle sprockets or gearing right to suit your weight, riding ability, and track conditions (You can thank me later). 

How To Choose Proper Gearing? First do an honest assessment of your weight and skill level. Generally speaking, beginning or heavy riders have a much harder time maintaining momentum and thus need good low to midrange powerbands to offer maximum acceleration between extremes. These riders are better suited with “Lower Gearing” motorcycle sprockets to allow the bike to stay in the “meat of the powerband” and offer maximum acceleration. Expert and top-level riders understand the importance of momentum and how to properly carry it thru corners & obstacles. In this case maximum top speed is essential, and these riders would chose a “Taller Gearing” for given track conditions. 

Next assess your track conditions and obstacles in which you ride. Review elevation changes, soil content, number of turns, and frequency of jumps. Tight tracks with deep soil or slippery conditions, and many turns or elevation changes are going to require “Lower Gearing”. Motocross tracks with fast sweeping turns, and long fast uphills are going to favor “Taller Gearing” to keep the midrange and top-end of the powerband working for these conditions. 

What Does Lower Gearing Do To The Powerband? Lower Gearing will allow the engine to move through the spread of gearbox quicker, but that also means the rider will be shifting faster and more often. The bike will hit or pull harder, and rev out quicker. 

When should you gear the bike down? There are several scenarios which demand lower gearing motorcycle sprockets. Engines produce less power at high altitude due to less available oxygen, and generally need to be geared down to rev freely. Tight tracks with short straights are better suited to lower gearing. High elevation changes or deep loam, mud, and sand will need more thrust which comes with lower gearing.

Evaluate the rider and determine if he/she is reaching the upper gears on the longest straights of the track. A motorcycle is faster at 3rd gear half throttle than 2nd gear full throttle. The bike will be more effective with a setup using 3rd thru 5th gears, than staying in 1st thru 3rd gear only. 

What Does Taller Gearing Do To The Powerband? Taller Gearing will produce engine power in a more linear fashion. The output is more controllable, and each gear will carry longer. While some hit will be lost, requiring more clutch use, the maximum speed potential of each gear will be increased. 

When should you gear the bike up? It is rare that a stock dirt bike needs to be geared up, however consider this option whenever more top speed is needed. This is typical for an expert rider when the track is hard, smooth, and flowing allowing momentum to be carried. This is also an option on more powerful bikes when the rider is looking for less hit in the lower gears. 

Small Steps With Gearing Changes: Always keep a log and typically never make more than a one-tooth change in gearing at a time. Carry a “Starter-Pack” of motorcycle sprockets which consists of a one-tooth lower countershaft sprocket and one-tooth, two-teeth, and three-teeth larger rear sprockets. By having this selection of motorcycle sprockets on hand, a rider will easily be able to change gearing at the track to suit the conditions. 

Motorcycle Sprockets Construction: Not all motorcycle sprockets are created equal as they come in different patterns, tooth designs, and materials. Hardened Steel is better for longevity and cost, while Aluminum is lighter. Anodized Aluminum is often a good compromise offering a hard outer layer for durability, while keeping weight and cost to a minimum. More expensive alternatives such as Stainless Steel or Titanium offer advantages in wear and performance. Always purchase a high quality Case Hardened, Steel front countershaft sprocket as this is a high wear item. Some rear motorcycle sprockets such as Renthal Twin Ring, and Supersprox Stealth offer a two-piece Aluminum Inner Ring with Hardened Steel Outer Ring design. This provides both weight and durability advantages, and combined with good quality motorcycle chains will last for many races. 

New Motorcycle Sprockets Mean New Motorcycle Chains: It makes no sense to change to fresh motorcycle sprockets if the motorcycle chains are wasted. Running a new sprocket with worn motorcycle chains costs power and instantly wastes the sprocket thru the improper interface. Do yourself a favor and always buy good quality motorcycle chains anytime you plan on installing new sprockets. Good quality motorcycle chains will drastically reduce the wear on your entire drive setup. 

Motorcycle Chains Construction: There are essentially two types of off-road motorcycle chains construction: Conventional and O-Ring/X-Ring designs. Conventional motorcycle chains use steel pins with a press-fit soft metal bushing to form each link. These chains use a mechanical seal to protect the link, meaning there is minimal clearance between the rollers and the link. O-Ring or X-Ring chains use a ring to seal lubrication in the link and keep dirt and water out. These motorcycle chains are much more durable, however they are more expensive and heavier. These chains are great from a maintenance perspective because they do not require adjustment as often or require as much cleaning as a conventional chain.

Proper Motorcycle Chain Adjustment: The condition and adjustment of the drivetrain has a significant effect on both the handling and engine performance of the dirt bike. The simple chain-and-sprocket is still the most efficient way to transfer power from the engine to rear wheel on a motor vehicle with 12+ inches of suspension travel. The forces transferred through the chain and into the suspension can have a very positive effect on handling. During acceleration, the chain forces the rear wheel into the ground. Motocross racers depend on the chain forces when pre-jumping and landing. Racers land from a jump with the throttle-on to provide more resistance to the rear suspension to prevent it from bottoming.

If the chain adjustment is too tight, too loose, or the wheel is not aligned in the swingarm an experienced rider will notice the difference. An improper setup or poor chain/sprocket condition can absorb as much as 5 horsepower at the rear wheel on the average dirt bike!

To set the chain adjustment first start with a perfectly clean chain to get a truly accurate reading. Next disconnect your rear shock to determine where the tightest point in the swingarm travel occurs. Adjust the chain to have approximately 0.5 inches of free-play at this swingarm location. This location is when the swingarm is parallel to the ground and the rear axle is at it’s farthest point from the swingarm pivot. Next ensure the sprocket alignment is correct. Use the swingarm markings as a guide but in production there are slight tolerance differences in these marking which can make a difference to the racer. Most race mechanics prefer to use an alignment gauge, which fits into the centers of the rear axle and swingarm pivot bolt to get this correct. The cost of the tool will easily be paid for with reduced bearing & drivetrain wear/replacement.

This article was picked up from an unknown user, if anyone knows the author let me know and I will give credit where it is due!

Related posts:

1. REAL TESTS! 2010 MXA RACE TEST OF THE 2010 YAMAHA YZ250
2. The Straight Dope: Which Handles Better, A Two-Smoke or a Thumper?
3. Chassis Setup – Handling and Sag

One of our passionate readers started circulating a petition to the AMA to allow two-stroke race machines into the Pro ranks. His suggestion was to allow a 144cc two-stroke to compete in the 250F class and a 300cc two-stroke to compete in the 450F class.

While it’s not the ideal many two-stroke fans would choose, it is a step in the right direction.

It’s important to keep one thought in mind, this is a step for the Pro Racing ranks. Many journeyman Pros have to pay for their own race efforts and spend quite a bit of cash in order to make their machines competive with the best in the world.

Of course when 144 and 300cc two-strokes are brought up the first question to come up is who makes machines in those sizes. This is a guide to point you in the right direction. It’s not as difficult as you would imagine.

Let’s start with the simplest, easiest path and move to the more labor and cash intensive.

Which manufacturers provide machines in these sizes?

TM Racing a small manufacturer in Italy builds purpose built machines in both a 144 and 300 size for motocross racing. These machines have been talked about for years, but have had spotty distribution in the USA for many years.

That has now changed since Barker Bros Cycles has taken over distribution rights for the US. In the first year of being US distributor, Dan Barker has placed machines in different parts of the country to allow potential customers to see and test the machines for themselves. In addition he provided Dirt Rider and Dirt Bike with a TM 144 MX for testing. Check out the current issue (August) of Dirt Bike magazine.

For 2011 look for TM Racing and Barker Bros Cycles to make bigger waves in the US market.

For those that don’t know much about these machines, they are hand built, works style machines. The best way to describe them is to say TM Racing is the Ferrari of dirt bikes. Don’t take anyone’s word for it, try one and decide yourself.

Purpose built 144 engine from TM Racing. These machines are super fast!

What other manufacturer build a 300cc two stroke motocross machine? The TM Racing 300 MX

Of course we can not leave the bright Orange machines of KTM out of a discussion of machines with different displacements. KTM builds bikes in many different sizes.

Over the past few years KTM has begun to make inroads into the sales strangle hold that the Japanese had enjoyed for many years. More folks are buying them, liking them and recommending them to others.

The 2011 KTM 150 (actually a 144) has taken accolades from every quarter with this machine.

While KTM does not build a Motocross specific 300cc motocross machine, they offer a 300cc kit for their 250SX. While it is an option it’s more expensive than buying on direct from the manufacturer.

Take the 250 SX and add the $800.00 300cc kit and you have a 300cc MXer.

These are all the parts included with the KTM 300cc kit.

MXA did a test using this kit on a 250SX. MXA KTM 300SX Big Bore

The lone Japanese manufacturer to still build and import two-strokes into the USA is Yamaha, with the YZ125 and YZ250. Unfortunately these machines have seen few updates since 2005.

While they are great machines, they are dated looking and have not had updates applied to them like the four-stroke line-up has.

Still they are good handling machines that can still be bought new from a dealer. That is if you can find a dealer that has any in stock! There seems to be very few of these machines built and made available to the buying public.

Even with these short comings, the Yamaha is a great platform to race in the Pro ranks.

To bring the little YZ125 up to the proposed 144 size there is a few options available. Probably the easiest is the GYTR 144 kit available directly from Yamaha.

The video below is of Pro Racer Max Anstie testing the GYTR kit…

While it’s not an inexpensive option, it certainly is one way of owning a YZ144.

MXA did a test on the GYTR 144 kit. MXA YZ144

With the YZ250 you could also build a 300cc racer using the L.A. Sleeve 270 kit. While this kit is built by using a steel sleeve, it seems to work quite well as the MXA test below shows.
MXA YZ 270 Build

In addition to the solutions listed above there are many companies that perform Big Bore surgery to your 125 and 250 two-stroke machines. Any one of these companies will take your bike and turn it into a 144 or 300.

My suggestion before trying any of these builders, is to ask others that have had work done by them and see what their experience was like. Then if you can talk them into it, try to take a test ride to see what you think.

Athena – Builds big bore cylinders for two-strokes. Available through larger distributors and your local dealer.

Eric Gorr / Forward Motion – Eric has been building 144 kits since 1976! He has tons of experience with big bore two-strokes. There is also some great advice to be found on his site.
http://www.eric-gorr.com/international144kits.html

Eric Gorr big bore kit.

Millennium Technologies – Well-known for there plating services, they also offer big bore services as well.
http://www.mt-llc.com/bigBoreKit.html

Kustom Kraft Performance – Another shop that has been in business for many years.
http://kustom-kraft.com/big-bore-kits.html

MAX Power – Builds big bore kits for all Japanese two-strokes
http://www.maxrpms.net/shop/index.php/motorcycle-kits/mk-yamaha/yz250.html?SID=078130f90c171feaf15eaa36f74c0d72

If you are determined enough to get something done, it can be done. There are machines available or that can be built that will fit into this proposed rule. Next up is to work to get this new rule passed!

If you have not done it, please sign this petition. http://www.gopetition.com/petition/38076.html

Related posts:

1. TM Racing Releases 2011 model line-up
2. German SuperMotard
3. Two Stroke AMA Rule Change Petition

When someone says they can run their two stroke at 200:1 the first thing that happens is that people’s eyes go wide and they say “Huh?” It happens every time the Project Two 50 team mentions they are running a 200:1 mixture. Of course there are many that are skeptical.

How can anyone possibly use such a lean fuel/oil mixture?

The folks over at MicroBlue Racing have the answer. Ten years ago founder Craig LeClaire developed a coating process that virtually eliminates metal to metal friction and wear. When you eliminate friction the need for oil becomes less important.

This process begins with superfinishing all parts before the coating is applied. This removes the peaks created during the manufacturing process, therefore changing the surface profile of the metal. It’s easiest to equate this with a rock versus a skipping stone, a smooth stone skips best.

Once the parts are superfinished, the MicroBlue coating is applied. This process occurs at the molecular level which can be verified by a Stereo Scope.

Because of this interaction, there is now no such thing as a lubricant starved condition. Which is why so little lubricant is required.

Craig say, “MicroBlue actually changes the way lubricants work. You know how slippery you feel in the shower with soft water? Soft water changes the way soap wets your skin, making you feel slippery. When a lubricant comes into contact with any MicroBlue coated surface, it does exactly the same thing-by changing the way a lubricant works on the surface.”

Initial tests of this technology on a two stroke were performed on a Ryobi leaf blower. Once the coating was applied to the cylinder, piston, rings, crank and bearings the engine was assembled, then tested.

The testing consisted of connecting the Ryobi Leaf Blower to a pickle bucket filled with fuel mixed at 200:1 Craig started the leaf blower in the barn, wired the throttle wide open and left the machine to run. This test was an attempt to seize the motor. Craig went back to the barn and amazingly the leaf blower was still running!! Total run time? 60 hours – non-stop!

The leaf blower was sent to a third party for testing. Where an additional benefit of running the lean fuel mixture was discovered, particulate emissions were completely eliminated!!

Of course this piqued the interest of the EPA. Because of the initial tests the EPA has authorized a SBIR Phase 1 Project to study and evaluate particulate reduction in MicroBlue Racing coated two stroke engines.

Next up, a test on a full sized two-stroke motocross machine. When Craig was introduced to Project Two 50, he recognized an opportunity to showcase this process to a wider audience.

After much testing, the Project Two 50 team is now running their machine at 200:1 The bike will make it’s Pro race debut at the Red Bud AMA Pro Motocross race on Saturday July 3rd.

If you’re a fan of two strokes, make sure to tune in to the first moto on Allisport.com and to watch the second moto LIVE on NBC.

http://www.microblueracing.com

http://projecttwo50.com

Related posts:

1. MICROBLUE Racing sponsors Project Two 50 -Results in Low Emission YZ250 Two-Stroke
2. Project Two 50 to Race Unadilla
3. Two-Stroke Fuel Injection is Here…. Now!

One the biggest supporters and sponsors of Project Two 50 is our friend Craig LeClaire from Micro Blue Racing located in Rockford IL. His ideas, suggestions and input have taken our simple idea and turned it upside down.

The original idea for Project Two 50 was to compete using a 250 two stroke in the AMA Pro National Motocross class against the 450 four-strokes. In itself an admirable goal.

Once Craig became involved the entire project changed focus. It has grown into a challenge to our entire team, a challenge to build the cleanest two-stroke, emissions wise, in the world.

As you could imagine, this is a very tall order, which involves many ingredients to achieve. The repercussions of achieving this goal are far reaching and in some ways life altering.

The thing is we actually did it!

How can this be possible on a normally aspirated two-stroke engine?

It begins with Craig and his insatiable thirst for improving the Internal Combustion engine. With a background in metallurgy and racing, it was a natural for Craig to begin experimenting.

What other “tuner” uses a high powered microscope to ensure all metal parts are perfect at the molecular level? While I don’t know that answer to that question, I do know that Craig does so with every piece of metal that he comes into contact with.

When you look at the parts from your engine in this light, everything changes. You see that a part that looks smooth to the naked eye actually looks like the dark side of the moon under the microscope.

Craig invented and developed a process that coats metal at the molecular level. This process is called MICROBLUE. Once you see a part that has been treated with MICROBLUE coatings you will never forget it. It’s blue color hue is distinctive.

The main benefit of MICROBLUE coatings are to reduce friction. When you reduce friction you increase power. This can be proved quite easily, by holding a MICROBLUE treated Ceramic bearing in your hand and spinning it. The first time I did this little experiment, my jaw dropped in amazement. It just spins and spins and spins.

Below are a few YouTube videos that show how the MicroBlue Bearings and coatings work on differing applications

Craig also applied the MICROBLUE coating to the moving parts of a two-stroke engine. In his first experiments he treated a Homelight Leaf Blower and performed a test. He connected a large gas tank to the blower, mixed up a few gallons of fuel, wired the throttle wide open and let the blower run for over 20 hours straight.

Under these conditions, you would expect that when you returned that the blower would have stopped running. But it was still screaming away when he returned!

Now to the important part, he mixed the fuel using Homelight oil at 200:1 Gas/Oil mix!!

A fabulous by-product of this test was the particulate matter (the part that affects emissions) was virtually eliminated!

When Craig joined us on the Project Two 50 team, one of his goals was to apply this technology to a two-stroke motocross bike. Which is what we’ve been testing over the past few months.

Our race bike uses a 100:1 Fuel/Oil mixture using Amsoil Dominator Synthetic. This fuel mixture accomplishes our goal of lowering the Project Two 50′s emissions to the cleanest seen in a production two-stroke!

There is still more testing to be done. One important step will be for our initial tests to be confirmed by independent testing. Which is on  schedule to be done as soon as possible.

For you tuner types reading this, I’d like to point out something for you to think about. Our MICROBLUE coated YZ250 engine requires a 210 main jet to run properly! The standard main jet used in a YZ250 is a 178!

In my mind there can only be one explanation for this change in jet size, this motor is pumping more air than a standard YZ250 two-stroke.

An interesting benefit is the sound coming out of this motor. It just does not sound like a typical YZ250. It has a deeper, more throaty bark. Another important point is the power delivery, which is very usable, with a tremendous mid-range that screams out into an amazing top-end. This is one very powerful machine. To top it all off it’s clean.

So to restate that a powerful, fast, clean and sweet smelling two-stroke!

MICROBLUE Racing has been using the MICROBLUE coating technology since 1999. Racers that understand the idea that friction reduction equals gains in horsepower are drawn to MICROBLUE. Racers involved in forms of racing from soapbox derby racers to Tractor pulling realize that friction reduction equals additional gains.

MICROBLUE Racing first entered Pro Racing in 2004. It is difficult to turn on the television in Motorsports without seeing MICROBLUE technology in action. It has been a race “secret” for many well-known racers and teams for many years.

You could say when it comes to Pro Motosports Racing, that everyone in the know, knows MICROBLUE.

Please take a moment to read the information in the “side bar” below. This is an independent test of the MICROBLUE coating using the standard Falex testing. This will allow you to take a glimpse at the potential of the MICROBLUE coating.

http://www.microblueracing.com/

Falex Testing of MICROBLUE coating

A solid film lubricant coating that has proven to be very effective in preliminary tests is the MicroBlue® tungsten disulfide coating. This coating is patented by Material Technologies, Inc. and is provided by the same company. This coating is applied by a low-cost atmospheric pressure particle impingement process. The coating produces almost no dimensional change (less than 1 micrometer) and is applied at ambient temperature. The MicroBlue® coating provides a low friction surface via two mechanisms. One mechanism is the inherent lubricity of the tungsten disulfide thin film on the surface.4 The other mechanism, which is perhaps more important, is that the tungsten disulfide acts as a wetting agent for hydrocarbon-based lubricants because of their affinity for the coated surface which causes the lubricant to more effectively wet the surface. The high durability of the coating is most likely due to the patented application process in which the tungsten disulfide is mechanically bonded to the surface by filling in micron-scale depressions in the surface.

To evaluate the coating durability, Aerodyne Research, Inc. (ARI) recently had block-onring tribological wear tests performed on standard and MicroBlue®-coated test specimens (Figures 1 and 2).

These tests dramatically illustrated the ability of the coating to provide low wear and friction, and in particular, demonstrate its ability to maintain a lubricant layer between surfaces under very high stress conditions. The tests were performed by Falex Corporation using their Blockon-Ring test apparatus. The test conditions were selected to be somewhat representative of a diesel engine crankshaft bearing under very high load. The test conditions were: 2000 rpm, 100°C, 150 lb force, and 100000 cycles (40 min). The Falex H60 block (SAE 01 Tool Steel, Rc 58-63 Hardness, Ra= 4-8 roughness) and S10 ring (SAE 4620 steel, Rc 58-63 Hardness, Ra= 6-12 roughness) were selected for the test. High quality diesel engine oil, Castrol GTX 20W-50, was used for the test in which the ring was partially submerged in the lubricant. Tests were performed for the standard H60 block and S10 ring as the control test, and for the same block/ring combination after being treated with the MicroBlue® surface treatment. Images of the specimens are presented above in Figures 1 and 2. The data sheets for these tests are presented in Figure 3 for the control test and Figure 4 for the MicroBlue® treated test.

The most impressive aspect of the two tests was that the standard (control) test specimens failed catastrophically in 5 seconds due to galling (see the note at the bottom of the test report in Figure 3) while the MicroBlue® treated specimens ran for the full 100,000 cycle duration of the test and showed minimal wear at the end of the test. The wear volume on the block for the standard specimens was 3.64 mm3 in 5 seconds, while the wear volume on the block for the coated specimens was 0.0354 mm3 in 40 minutes – 1/1000th the wear volume in a 500x longer test. Clearly, the severe conditions prevented a lubricant layer from being present in the interface between the block and ring components for the control test (a boundary lubrication condition), leading to immediate galling. For the coated specimens, on the other hand, the MicroBlue® coating’s interaction with the lubricant evidently maintained a thin lubricant layer between the parts to prevent galling and greatly reduce wear (mixed or mixed+elastohydrodynamic lubrication regimes).

While the conditions for these tests were very severe, they demonstrate the important aspects of the coating – its interaction with the lubricant to maintain a lubricant layer between parts and nearly eliminate wear, and its very high durability as demonstrated by its survival under these severe conditions for 100,000 cycles and showing minimal wear at the end of the test.

Related posts:

1. Can a Two Stroke run at 200:1 fuel/oil Mixture?
2. Titanium Nitride for the Factory ride
3. Testing time for Maico

Here is the story of my rebuild of my 2005 KTM 200 EXC, twostrokemotocross.com inspired me to have a go and do it myself.

Since starting racing as a 10 year old in 1984, I have had two dirt bike ambitions I really wanted to fulfil, first to ride a 500, second to rebuild my own motor. Well the first was ticked off last year in the dunes at Lancelin, north of Perth, Australia, thanks to the kind Kiwi who said in response to my request for a photo of his gleaming 2001 model CR500, “Take it for a spin if you like.” Needless to say I took the offer up, let me tell you holding that baby wide open in 5^th in a pair of shorts and sandals was quite a thrill. I have a neat little scar on my left calf to remind me of when the curve in the pipe branded me on that great day! Anyway, after 5 heavenly minutes I figured I would be wise to hand it back while I was still in 1 piece.

On my return to England I was reunited with my 200, which has since seen another year’s hard use. The routine I’ve been following is do the top end myself every 100 hours, get someone else to do the mains every 200hrs. Almost immediately after the 200hr full rebuild I drowned it good and proper, it seemed like I pumped a whole gallon of brown water out of the old girl, which was pretty depressing, but she seemed to shrug it off, but when I recently drowned it again in a huge mud hole at the 380 hr mark, I figured it was  time for a strip down. When the next day I landed off a log and there was a horrible squealing noise that lasted about 30 seconds my mind was made up. Later in the day it boiled over when I had to scream it to get up a sandy climb in a wood, so I knew I had to get to work. I had followed the CR 250 rebuild on the site with interest, and done lots of google searches about splitting crank cases, so armed with my workshop manual I set to work.

I bought a KTM clutch holder and primary gear holder as well as the inner ring tool, for removing and fitting the bush that slips on the crank and sits in the roller bearing. As their rotor holder is $160-00 here, no joke, and the shift roller holding key was out of stock, I asked my friendly neighbour Phil the farmer and his cousin Steve if they could help me out and 1 hr later I had a DIY rotor holder and key blank. The blank took about 20 minutes of filing to get a good fit and I was good to go.

I opted to take the motor out before removing the primary gear nut and clutch hub nut, they both came off with the motor sat on the bench, with me holding the motor in 1 hand and a breaker bar in the other with no problems. I did remember the primary gear nut is left hand thread! At this point I discovered the source of the horrible noise, the intermediate starter gear had spat a tooth off, which had done some pin ball action before shredding the water pump drive. New parts were duly ordered.

This motor doesn’t require a crank case splitter , the cases eased apart with no problems, I tapped away gently with a plastic mallet and used the tabs on the cases to prise them apart by hand. I was a bit concerned about putting the tranny back together so I took lots of pictures and made running notes as I disassembled it. Actually there was nothing to worry about, the important thing is to make sure the stop discs on the end of the shafts don’t get forgotten, and keep a grip on the lower end of the whole assembly when removing it so gears don’t come flying off.

I had decided because of all the water that’s been in the mill I would replace all the high speed bearings while I had it in bits, I opted to leave the gear shift and kick start roller bearings and the shift drum bearing in, as they showed no signs of corrosion and don’t lead too hard a life. I was pleased to see that the cylinder looked just as it did 100 hrs before, you can still see all the original hatching marks, and there isn’t any sign of wear. The piston with over 100 hrs on it had just a few signs of blow by.

I took the crank and rod assembly to my KTM dealer for assessment, using the time honoured technique of grabbing hold of the rod and giving it a damn good yank in two directions, mechanic Nick determined that the big end was at the end of its life so fitted a ProX kit, considerably cheaper than the factory parts and just as good so they say.

With that sorted and a bag of new bearings in my possession it was time to get the old bearings out and stick the new ones in. At last I would have a 2 stroke engine in pieces on my kitchen table! I started out by applying localised heat with a hot air gun, this worked fine for the main shaft and countershaft bearings, but the crank bearings were much tighter and I didn’t want to start whacking them too hard.

Next I put the cases in the oven for about 5 minutes then tried again. It is possible to get a socket on the inner race of the ball bearing and drift it out from the outside in, this worked fine without the need for too much force. I had the case sat flat on a large wooden chopping block. Next up was the roller bearing side, there is no clearance to get a socket on to the bearing on this side, so I used a drift and worked my way around the bearing, hitting the ends of the rollers. Several  rollers quickly fell out, so I was then able to hit the bottom of the race. Not pretty but it worked fine. I think for next time I will make a draw bolt and sit a big piece of plate across the case and pull the bearing out from the inside to avoid having to hit anything.

Installing the bearings was straightforward, I heated the cases up and dropped them in, the crank shaft ball bearing started to go I out of square but the lightest of taps got it out again and I got it right the second time. I made sure they were all the way home by tapping in with a socket on the outer race and went back up to the workshop for reassembly.

The workshop manual instructions were very clear, and I got the gear assembly in to the right hand case at the second attempt, it was a little bit fiddly. Putting the shift forks and drum back in was straightforward. Then it was time to stick the left hand case on. I sat the right hand case on a big pile of rags to allow for the shaft end sticking out and popped the left side on. I tapped away gingerly with the mallet again and got it seated without difficulty.

Then I put the bottom end back in the frame before replacing the primary gear and clutch hub. At this point I have a confession to make. Following the details in the manual, I torqued the primary gear nut to 180NM using a pretty gnarled old wrench I’d borrowed from Phil, it was only later, looking up the engine mounting bolt setting in the owner’s manual that I noticed the figure for the 05 model is 130 NM. The workshop manual was written in 02 and I didn’t check for revisions. Lesson learned.

I figured too tight could not be a good thing so whipped the clutch cover off and slackened it off then went to torque it to 130NM. I got to the point where I was thinking  the wrench should have clicked but it hadn’t, so I slackened it off and inspected the thread on the crank and the nut, to my horror there was damage to the thread and not the nut, but it was fairly slight, and with a 4 day riding trip to France 2 days away I had no choice but to stick it back on. From the amount of force I  was applying I was pretty sure there was over 100 NM of torque so just had to cross my fingers. Maybe the wrench was under reading and that initial go at 180 started the damage, the nut certainly went on square, I put it on with my fingers to start with so it wasn’t cross threaded. I hope that next time I pull it  apart it can be fixed on a lathe, going a size down on the nut.

The rest of the rebuild was a routine affair, so with the whole bike back together I prepared to fire her up. I had, of course, turned it over by hand at every stage of the build to check nothing was amiss. Well it’s fair to say I have never heard so many strange noises from the motor, I was a bag of nerves as I warmed it up. A friend pointed out that it’s only because I was listening so hard! It turned out the noise that really freaked me out was the very bashed about pipe touching the bash plate and creating a harmonic at mid rpm. I’ve put about  40 hrs on it since and it’s running perfectly, so I think I’ve got away with it.

At the same time I set about a much needed fork service, and took the shock to the shop for a rebuild, the seals were completely shot and the whole bike felt like an old sofa. I purchased a motopower fork service dvd which was brilliant, I made notes as I watched it in the house then went up to the workshop and set to work. The process was easy and straightforward, instead of a seal driver I used a piece of 6inch roofing lead rolled around the slider with a nice smooth edge filed on to it instead and it worked fine.

At this point I have to make confession number 2, my note taking failed me and I put the seals in the wrong way round. D’oh! What I know as a result is that over 4 days riding each fork leg lost 250ml of oil, and I got a good insight in to the effect of an increasing air gap on the feel of the whole bike. As a result I’m running and 10mm larger air gap (120mm instead of 110mm) and for the trail it’s perfect. I can also vouch for the fact that the seals work a whole lot better installed the right way roun! They were undamaged as a result of being in the wrong way round.

I really enjoyed the whole process, I got a huge sense of satisfaction and achievement from doing it, and it confirmed to me that it is indeed true that you can service your 2 stroke at home with some basic skills and a decent tool kit. Sure the bottom end needs doing by the shop because the crank has to be balanced after driving the pin out, but this is not an expensive job. I saw on Thumpertalk a guy talking about his son’s KX80 with 1800 hrs on it, so hope I can maintain my bike in good condition for many a year to come. I only have to look up clips of the World Enduro Championship from 2005 on utube to remind myself that it’s me that holds the bike back, not the other way round, if I ever get a bit of new bike envy. The motor is bog stock, the suspension has been revalved and sprung for my weight and I have a Scotts damper fitted which has really calmed the whole thing down. I ride some rocky terrain up here in northern England and never ever felt comfortable at speed on rocks until I fitted it. It is a great piece of kit. An Enduro Engineering seat recently replaced the original which was getting seriously uncomfortable over the course of a day and is a big improvement.

It sits me slightly higher than the original which I prefer too. I also have a 12.5 litre tank. Overall I love the package, it’s super light, plenty fast enough, and always fun. One of our mags over here always complains about the fierce power delivery of the 200, saying you never know if it will wheelie, spin or drive. Well excuse me, get used to it! That’s my idea of fun, why would I want to ride something with  a dull, linear power delivery. My bike is more challenging, sure, but more rewarding. I had a go on a friend’s 04 WR250 and never want to repeat the experience. I think I’d have had more fun pedalling. Maybe the tester doesn’t know what the clutch is for!

Related posts:

1. Engine Build HPP Valve Honda CR250 – Part 3
2. Engine Build HPP Valve Honda CR250 – Part 2
3. Engine Build HPP Valve Honda CR250 – Part 1

This is part nine and the final part of the CR250 engine build and will cover head, reed valve and carburetor installation. I will also perform the initial leak down test.

Tools needed: Torque wrench, leak down tester, metric tool assortment

Apply oil to the six cylinder head studs and install them into the cylinder. Torque the studs in three steps and in a criss cross pattern to 9 lb.ft.
Again use a stud tightener or two opposing flange bolts to properly torque the studs.

Place a new head gasket with the “up” mark facing up and aft

Install the cylinder head and apply oil to the cylinder head studs and landings.

Install the six cylinder head nuts and hand tighten. Torque the nuts in three steps and a criss cross pattern to 20 lb.ft.

Install the reed valve with a new gasket into the cylinder

Install the intake manifold with the intake oriented to the left. Apply oil to the six intake bolts and install the bolts along with the throttle cable
retainer. Torque the bolts in three steps and in a criss cross pattern to 7 lb.ft.

Now is a good time to perform the initial leak down test. Cycle the piston to bottom dead center (BDC). Install the spark plug and torque to
13 lb.ft. Install a exhaust plug to seal the exhaust outlet. Attach the tester to the inlet and seal with a hose clamp.

Pump the tester to 6 psi and start timing. An acceptable loss rate is 1 psi/min. Less is better.

Remove the test equipment and the exhaust plug. Install the carburetor aligning the tab on the carb intake bell with the notch in the rubber intake boot. Tighten the intake clamp snugly and give yourself a big thumbs up for completing a total engine build.

Related posts:

1. Engine Build HPP Valve Honda CR250 – Part 8
2. Engine Build HPP Valve Honda CR250 – Part 3
3. Engine Build HPP Valve Honda CR250 – Part 7

This segment will cover piston, rings and cylinder installation and power valve adjustment.

Tools required: Torque adapter, torque wrench, standard assortment of  metric tools.

Most people have rebuilt the top end of their ride so a lot of this information is old news.  I am presenting the techniques that have worked best for me over the years.  Hopefully you will see something that might be helpful the next time you tackle this job.

First order of business is protecting the crankwell from debris.  Stuff a shop cloth in and around the connecting rod.

Apply oil to the cylinder studs and install with the rounded portion of the stud facing down.  Use either a stud installer or a couple of nuts tightened together and torque to 9 lb.ft.

Coat the small end bearing with two stroke oil and install it into the rod.

Install a circlip into position on the piston.  Coat the wristpin with two stroke oil.  Place the piston over the connecting rod orienting it with the skirt cutout facing aft (If you are using an OE piston, orient the piston with “IN” facing aft.

Install the other circlip insuring both are correctly seated in their grooves. Also, never re-use circlips, you’re just asking for trouble for a dollar’s worth of parts.

Apply two stroke oil to the piston rings and ring landings.  Orient the rings with the mark (next to the gap) facing up and install with the ring gap straddling the pin in the ring groove.

Apply oil to the two dowel pins and install.

Install a new base gasket.

Apply grease to the pinion shaft and make sure it is oriented correctly.

Liberally apply two stroke oil to the cylinder wall.  Place the cylinder over the studs with one hand while compressing the top ring with the other. Carefully lower the cylinder over the top ring then compress the second ring and repeat.  Once the cylinder is past the rings, be careful not to turn the cylinder anymore than absolutely needed to orient the cylinder over the studs – It is very easy to snag a ring on one of the ports and break it.

As you carefully lower the cylinder, take a look under the right front corner and “eyeball” the line up between the upper and lower power valve pinions, you may have to rotate the power valve clockwise slightly to get a good join up.  The pin on the lower pinion goes in the gap in the upper pinion. The cylinder should seat without any gaps.  If you have a gap, chances are the power valve pinions aren’t joined correctly.  Simply lift the cylinder an inch or so and manipulate the power valve linkage as you lower the cylinder again.

Once the cylinder is firmly seated with no gaps, give the power valve linkage a tug to make sure it doesn’t move.  Apply oil to the cylinder studs, install the four flange bolts and finger tighten.  You will need the torque wrench adaptor to tighten the front two nuts.  Please do not use an open end or even a box wrench and guestimate the torque.  Unless you have been doing this a long time and have a calibrated hand, you are setting yourself up for a blown base gasket or possibly a warped cylinder base.  Buy the adapter and do the job right.

Tighten the flange bolts in a crisscross pattern in three steps.

Using the kickstarter, cycle the piston a few times.  It should move freely and quietly.

Next up is the power valve adjustment.  Now that the power valve is connected to the governor, the slack in the linkage can be adjusted out.
Using a 4mm hex bit on a socket wrench or a T-handle (something you can generate some torque with), loosen the socket bolt on the pinion shaft slightly.  Turn the power valve fully counterclockwise and insure the flap valve is fully closed (check the reference mark on the left side inspection port points to “L”).  With everything fully closed, rotate the pinion shaft slightly more to the left with the 4mm hex bit and tighten and torque to 4 lb. ft.  The power valve linkage is now slightly preloaded and their should be no slack in the linkage.  Honda recommends 0-0.5mm gap between the pinion spring and pinion lever for this setting.  I like to set mine right at 0.5mm.

Check that the power valve position indicator is still pointing to “L”.  Apply two stroke oil to a new O-ring and install the left side cover and torque to 9 lb.ft.

I like to spray some Permatex gasket adhesive on gaskets that don’t have alignment dowels.  Just a few quick sprays of Copper spray in this case.

Let it tack up a couple of minutes and position the gasket right where you want it.  This spray is available in Copper (hot areas) and silicone and is also good insurance for areas like center case gaskets.

Install the right side power valve cover and torque the bolts to 9 lb.ft.

Starting to look like a motor now.

Related posts:

1. Engine Build HPP Valve Honda CR250 – Part 9
2. Engine Build HPP Valve Honda CR250 – Part 7
3. Engine Build HPP Valve Honda CR250 – Part 5