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The Philosophy of Carburetor Tuning

by Big Bore Maico on 08/19/2008

I hope everyone who reads these pages can take something from them that makes their dirt biking more fun, with less hassle and expense.

Iíd like to start out with the definition of a word. POTENTIAL. The definition of the word is as follows; existing in possibility, capable of development into actuality! The reason I started here is because motorcycles have potential. Potential to have fun, and also to hurt and kill you. So we must understand the risks involved in whatever endeavor we wish, particularly when a helmet is required. So in dirt biking, we have, existing in possibility, the capability to ACTUALLY HAVE FUN. There are various and sundry reasons why we donít have fun, as that potential is there as well. So with that frame of reference in mind, letís start.

As you are on the Two Stroke Militia web site, you can rest assured that we will compare and contrast the differences between Two and Four stroke cycle engines. And as more articles are written we will build on this foundation, of potential for fun.

As you look at your dirt bike, sitting on the stand, cleaned and HIGHLY maintained, you smile, kmowing that soon you will be enjoying one of the ultimate freedoms of life. A feeling almost impossible to describe. Itís very difficult to explain to someone who has never ridden a motorcycle how much fun it is. And while you are looking at your bike, notice the fuel tank. A simple storage container really. It has a filler on top. And a valve on the bottom. They hold from 2 to 3 gallons of fuel. Or more correctly stated POTENTIAL! Inside this simple storage container is what separates motorcycle riders from bicycle riders. Yes we have the engine, but with out the fuel tank (and itís extremely volatile contents) we would have nothing more than a glorified overweight bicycle. What we, as dirt bike riders need to do is figure out how to take the potential that is stored in the fuel tank and convert it to FUN. Thatís where the engine fits in.

Inside your fuel tank sits a concoction of various hydrocarbons, along with modifiers such as detergents and elements that raise the octane rating, make your bike easier to start in the winter and less prone to vapor lock on hot days. The latter is rare on dirts bikes but it is a possibility. This is controlled by the refinery and is dictated by an engineer who spent many years learning so we can participate in the sport we so enjoy. What we need to understand is how to take the fuel, which has potential and make it useable. In liquid form, gasoline will NOT burn. It must be atomized into small particles and combined (homogonized) with oxygen to be burned in the combustion chamber. Thus, the air/fuel mixture is converted to heat energy during combustion. And that combustion process is what we are looking at.

Carburetors are one of those things that appears to be difficult and complex. But they are actually relatively simple. Others (myself included) consider them nothing more than a controlled leak. These devices are the key to unlocking the potential of ANY internal combustion engine. It has to mix the air and fuel together, in a unifom mixture, relative to engine speed (throttle opening) and it has to be adjustable. I will cover the basics here. There will be more to come.

From the tank, fuel enters a line that is controlled from a valve called the petcock. I said this will be basic. The fuel line is hooked to the carb, usually on one side, although some have dual feeds. Inside is a needle and seat. This little valve is what controls fuel flow into the carb. Itís generally two peices the needle and the seat. There are literally hundreds of variations on these, but they all function the same. Again, generally the needle is male and the seat is female. The seat is mounted into the carb body and the needle fits up into the seat. The needle literally rides up and down in the seat. It typically has a rubber tip on one end, shaped like a cone. That end goes up into the seat where a cone of the same angle is machined. The other end of the needle is hooked to the float. The float sits in the float bowl and has either a pivot or pins that it slides up and down on. As the fuel is turned on, the float is sitting on the bottom of the bowl. This is assuming little or no fuel in the bowl. The needle, typically fastened to the pivot side of the float, is hanging down, out of the seat. The fuel actually flows through the center of the seat and starts to fill the float bowl. When the bowl has reached a preset level, as determined by itís relative position to the needle, the float rises and pushes the needle into the seat and shuts off fuel flow to the carb. Here is where your tune up should begin. Weíll cover that in the tune up article.

From there are two basic circuits the fuel must go. They are the pilot jet and the main jet. In reality, MOST dirt bike carbs are two circuit carbs. The pilot jet being the low speed (partial throttle opening) circuit and the main jet the high speed (wide open circuit). Other factors include the throttle slide cut out, the air screw and the jet needle. The needle could be considered the most misunderstood piece in a carb. In effect, the jet needle, which is taperd, goes down in the main jet and is fitted through a hole in the slide. As the slide goes up and down, the jet needle changes position in the main jet. All those circuits, or ďsystemsĒ if you will affect the other. You can rarely change one without effecting the other.

Depending on throttle opening, engine speed and even Miles Per Hour (road speed), the air flow through the carb dictates the fuel flow. I will go into greater detail later on each individual circuit later. After the fuel has been metered through the various jets, it is introduced into the air stream. From there (in two stroke operation) typically the air/fuel mixture goes past the reed valve, into the crank case. As the piston descends on the compression stroke, it causes a pressure in the crank case. The reeds snap shut, closing off incoming air and fuel. Now we have a closed system. As the piston descends lower in the stroke the pressure becomes greater until the transfer ports open. All the pressure built up in the crank case blows the air/fuel mixture into the combustion chamber, where it can be, by spark, lit and cause the air/fuel mixture to be converted to heat energy. And the whole process starts over again. Thus the fuel has released itís potential, in the form of heat, and the engine converts the heat energy into mechanical force to drive (in this case) your dirt bike. Which equals GREAT FUN!!

90% of dirt bikers rarely race on a regular basis. For myself, itís the occasional GP, once in a great while a MotoCross and a vintage race every now and then. For most of us, itís a time for family and friends to get together and enjoy the outdoors, preferably while on a dirt bike. Generally, we are honest, hard working men and women (every box has itís bad apples). I base my motorcyle experience on what my fiends and I call the ďGrin FactorĒ. Not every ride is perfect. Equipment breaks, people get hurt and thatís not fun. But mostly, when Iím done with my day, if Iím grinning ear to ear, it was a success. And the majority of the time Iím grinning like a Cheshire cat. The hope of Two Stoke Militia, through these articles, is that your ďGrin FactorĒ is increased. And your motorcylcing experience is safer, more cost effective (essentailly more bang for your buck) and fun with your family and friends.

Please feel free to email Two Stroke Militia with ALL comments, good, bad or indifferent. We would love to hear concerns, and kudos if you have them.

More articles that build on this one will be coming soon. Check back often, and send everyone you know to Two Stoke Militia. Letís keep working towards equality for Two Stroke dirt bikes.

Always remember thisÖÖÖ.REAL MEN PREMIX.

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