Adapted from the 2012 test of the YZ125 featured in Motocross Action. I think we all saw a need for this recently."There will come a time when every manufacturer, race promoter, rider and AMA functionary will rue the day that the AMA allowed 250cc four-strokes to compete against 125cc two-strokes (not to mention the stupid unleaded gas rule that administered the coup de grâce to the 125 two-stroke on the AMA circuit)."2005
The 2005 Yamaha YZ125 is the acorn from which the modern YZ125 tree has grown. The 2005 YZ125 engine was all new (sharing only the kickstarter and clutch plates with the 2004 engine). It was smaller, lighter and faster. Yamaha ditched the domed piston for a flattop piston. They shortened the connecting rod by 3mm. They raised the exhaust port 0.5mm. They tilted the engine forward 7.5 degrees so they could straighten the intake tract from 16 degrees off-center to 5 degrees, which was a big deal. The coolant port on the head was changed because of the engine tilt. The water-pump impeller was changed to plastic, and the shaft was 4mm smaller. The clutch actuator arm was relocated inside the ignition cover. The transmission went from five to six speeds. The exhaust pipe got a straighter initial section.
The 2005 YZ125 got the plug-and-play aluminum frame. There wasn’t a single tube on the 2005 frame; instead, it used forgings, castings and extrusions. The 2005 Yamaha YZ125 frame was 4-1/2 pounds lighter than the 2004 steel frame. The bike weighed 197 pounds and got Honda-style brake routing and 7/8-inch Renthal aluminum bars for the first time.
The Kayaba forks were copies of Showa’s Twin-Chamber forks. We called them “Showabas.” The Showa features were the dual chambers, top-mounted compression adjuster and one-piece outer tube. 2006
On the 2006 engine, the shape of the combustion chamber’s squish band was radiused out to the edge of the band. The carburetor got one less bypass port and a new 6BFY42-74 needle. A stiffer power-valve governor spring made the valve open at 9500 rpm instead of 9000 rpm. The ignition got a new CDI mapping and an NGK BR9EVX platinum-alloy plug.
The greatest change in YZ history came in 2006 when Yamaha introduced its SSS suspension components to replace 2005’s Showabas. With SSS (Speed Sensitive System), Yamaha’s suspension went from 30-percent speed-sensitive damping to 90-percent speed sensitive. As for the shock, it was as close to a works shock as any production bike had ever come. It had an 18mm shock shaft instead of 16mm, Kashima-coated internals, a 30-percent-larger reservoir and a titanium shock spring.
Additionally, for 2006, Yamaha added adjustable handlebar mounts, a sleeker front-brake lever, an on-the-fly quick-adjuster clutch lever, wider fork-tube spacing, a low-profile rear-brake caliper, new rear brake-pad material, plastic rear-brake caliper and rotor guards, a softer and taller saddle, a new front number-plate shape, and a Dunlop D756 rear tire to replace 2005’s D739, but the front remained a D739.2007
Yamaha made five changes to the 2007 YZ125.
(1) The fork stanchion tubes were tapered into a thinner midsection, and new damping was added.
(2) The steel components on the Kayaba shock (spring guide, clevis bushings, rebound clicker internals and compression adjuster) were replaced with lighter aluminum pieces.
(3) The 7/8-inch Renthal handlebars were replaced with 1-1/8-inch Pro Taper Contour bars.
(4) Aluminum bolts replaced the previous steel bolts on the subframe, engine mounts, seat, side panels, front fender, triple clamps, rear brake caliper guard and chain guide. The front axle nut was changed to aluminum.
(5) The silencer’s perforated section was shortened by 75mm (with the remainder of the core constructed from non-perforated stinger) to improve throttle response without increasing sound.2008
For 2008, Yamaha made eight changes to bring the YZ125 into closer compliance with the YZ250F four-stroke. The 2008 YZ125 got the front brake caliper, fork stanchions, lower fork bracket and chain guide from the YZ250F. A new reed valve was added. The front master cylinder piston was made 1.48mm smaller, while the brake lever was positioned higher on the master cylinder and 9mm farther from the handlebar.
There were some notable weight savings in that the front brake caliper was 220 grams lighter, the wave-style brake rotors were 140 grams lighter, and the fork stanchion tubes were thinned again (and weighed 200 grams less).
The Kayaba SSS fork spring rate was upped from 0.41 to 0.42 kg/mm, and the fork stanchions were made 4mm shorter. 2009
After getting eight changes the year before, the 2009 YZ125 only underwent four changes for 2009.
(1) Yamaha dropped its oversized, steel, serpentine brake-hose holder in favor of a 32-gram-lighter aluminum clamp.
(2) For 2009, the chain got a special zinc coating.
(3) The textured seat cover of 2008 was replaced with a woven seat cover.
(4) The D739 front tire was dumped for a Dunlop 742F.2010
The 2010 YZ125 got new tank graphics.2011
Yamaha made several changes to the 2011 YZ125 model to make it conform to the world-wide global spec, so that they could sell the same model in every country. This is a money saving move, not an improvement. The global-spec changes were limited to a 75mm-longer silencer, with its core increased from 27mm to 30mm, and 71.5 percent more glass wool packing. Additionally, the 410 mainjet was exchanged for a 430, while the 6BFY42-3 needle was swapped out for a 6BFY43-3 needle (the needle and jet came as options in the tool kit in 2010). 2012
New tank graphics and a white rear fender.MXA Jetting Specs (2012 test)
Yamaha has their jetting down, and the 2012 YZ125 ran clean. Here are the 38mm Mikuni TMX jetting specs:
Pilot jet : 40
Clip: 3rd from top
Air screw: 2-1/4 turns out
Notes: Yamaha includes one richer (440) and one leaner mainjet (420) with the bike. The current 6BFY43-3 needle is a half-clip richer than the 6BFY42-3 needle used before the global spec came into play. MXA Fork Settings (2012 test)
Here is what the MXA wrecking crew ran for hardcore racing.
Spring rate: 0.42 kg/mm
Oil height : 340cc
Compression: 11 clicks out
Rebound : 6 clicks out
Fork-leg height: 5mm up
Notes: Way back in 2007, the YZ125 came with lighter, 0.40 kg/mm fork springs, which are better for many riders under the 150-pound mark.MXA Shock Settings (2012 test)
For hardcore racing, we recommend this shock setup.
Spring rate: 4.7 kg/mm
Race sag: 98mm
Hi-compression: 1-1/2 turns out
Lo-compression: 13 clicks out
Rebound: 10 clicks out
Notes: The high-speed compression adjuster is sensitive, so make small (1/8-turn) changes. It’s also a very important adjustment for keeping that rear wheel settled and for controlling wheelspin on hard, choppy terrain.MXA's First Mod
Before you buy a pipe, port the engine or slip in aftermarket reeds, gear the YZ125 down. Adding one tooth will perk up second gear, get you to third gear sooner and make the overall ratios between the six gears more user-friendly. (This means add a tooth to the rear sprocket)2012 YZ125 vs. 2012 125SX
It makes 34.01 horsepower. How does that compare to the only other 125 from the “Big Five,” the 2012 KTM 125SX? Surprisingly, for an engine that hasn’t seen a serious mod in six years, the YZ125 holds its own. While it never beats the 2012 KTM 125SX at any point on the curve (and actually gives up several horsepower to the Katoom from 6000 rpm to 8400 rpm), from 8500 rpm, the YZ125 sticks with the SX. The 2012 YZ125 makes 34.01 horsepower, while the KTM peaks at 34.69.
In comparison to the 2012 KTM 125SX, the KTM is faster and torquier, while the YZ125 has better suspension. We think the handling nod goes to the KTM, but the YZ125 is a bulletproof design, thanks to six years of refinement. Which would we pick? It depends on how rough the track is.2009-up YZ250F Rear End Swap
The rear end is a complete bolt-on swap. Assuming you have the Swingarm, Hubs, Linkage, and Brake Carrier. It offers the following over the YZ125 setup:
(1) Swingarm. The new YZ250F and YZ450F share the same hydroformed swingarm, which changes the rigidity balance and flex. It is 12 ounces lighter than last year’s swingarm.
(2) Rear hub. Another shared technology with the YZ450F is the new rear hub. It’s more compact, weighs nine ounces less than the 2008 hub, uses a 25mm diameter axle (compared to 22mm), and has three smaller wheel bearings (compared to two).
(3) Linkage. The shock linkage has been redesigned to conform to the new swingarm. The new low-rider link is 0.8mm longer than the previous center-mount version, while maintaining the same rising rate. The biggest change is in the rocker arm, which is 8mm longer.
This setup was used in Jessica Patterson's GYT-R YZ144, and Doug Dubach's YZ250. It gives some horizontal flex which apparently is amazing in whoops.UFO Restyled bodywork
UFO now makes a kit for just over $200 that changes the look to the "arrow" design of the foopers:
I hope everyone enjoyed this!