For a video on the construction of this YZ125 project, click on the image.


It’s exciting and discouraging at the same time. The Yamaha YZ125 has resurfaced, thanks to its new-for-2015 look. It’s finally getting some respect. In fact, it’s been called the best-handling dirt bike in the world. If that’s so, what have we been doing for the last 10 years? Running in place?
Short answer: Yes. Long answer: Handling is such a subjective term that it verges on being meaningless. The YZ125 is incredibly light and has less peak power than any full-size motocross bike currently sold. Those two factors make it a blast to ride, but don’t necessarily make it competitive as a race bike. So what do you do with it? Another short answer: Have fun! The purpose of this project is to have no purpose. This YZ125 was set up to ride anywhere with anyone. Call it an off-road bike if you want; we’ll just call it freedom.

Any bike has some limitations. The biggest one for the YZ125 is that it’s not officially an off-road bike, as viewed by the government. Translated, that means it can’t get a California green sticker and doesn’t have access to some public lands out west. We keep fearing other states will enact similar restrictions, but for the most part it hasn’t happened. Our idea was to tear down the remaining walls around the YZ with a 14-year-old trail-riding kid in mind. First things first: it needs more range. That’s easy. IMS has a 3.2-gallon YZ tank that hasn’t changed much in years. The good news is that it still fits, even with the changes in 2015.
More than anything else, we wanted to make this bike more crash-proof. The term “14-year-old” is another way of saying that this bike was going to get utterly thrashed. That’s the true wonder of the YZ125. It’s hard to break through willful misbehavior, even when crashes are involved. It’s kind of like when a cat falls out of a tree. He shakes his head and goes back to business. If a cow falls out of the same tree, different story. Crash-proofing the YZ just means common-sense bolt-on items. We installed radiator braces, with a skid plate and an Elite clutch perch, all from Works Connection. Cycra has Rebound handguards that are fairly hard to break because they’re spring-loaded.
We used Rocky Mountain ATV’s in-house Tusk wheels because we’ve found that they’re surprisingly tough and still affordable. The rear is an 18-incher and uses a primary drive O-ring chain, also from Rocky Mountain. It will never wear out.
Even the motor was put together with the most indestructible stuff we could find. A complete Hinson clutch would have been a little overkill, but the YZ needs a little reinforcement in that area. We gave it Hinson plates and heavy springs. It got a Uni Filter element and CV4 radiator hose, which are tough to poke or wound.

With 125s, the siren’s call of horsepower can leave you smashed and broken on the rocks. We know; we’ve hit that reef many times. The methods available all have “yes, but…”
Yes, you can increase compression, but you’ll probably have to run race gas. Yes, you can go to a big-bore kit, but you’ll lose rpm, plus make the bike vibrate and hard to start. Yes, you can turn to some porting wizard, but reliability goes out the exhaust port, along with a ring and part of the piston. A Yamaha isn’t as fast as a KTM 125 or 150, so deal with it and move on. The YZ still handles better and is more fun to ride. The only performance modifications that make sense given our do-anything mission are very basic. We installed a Vertex piston, which is state-of-the-art for two-stroke pistons. We put in a Moto Tassinari V-Force reed and installed an FMF Fatty pipe with a Shorty silencer. Those bolt-on mods will give you a full horsepower or so without negative consequences. From there, it was just a matter of dialing in the suspension for our 135-pounder. As it turns out, the newest version of the YZ is a little stiffer than it was in the old days. The fork is mechanically the same as the one on the current YZ250F, aside from the front axle carrier. The 125 still uses the older wheels and smaller axle. MB1 softened things up for trail riding, but not so much that it couldn’t answer the call of moto when needed.
Honestly, a YZ125 is such fun that the biggest fear is messing things up. But, this approach was so conservative that it made for a great result. The riding public can’t get its collective head wrapped around the fact that a 125 is a great trail bike—not just for kids, but for everyone. The weight and handling make the rider feel like Superman. They have their limitations, certainly. Hill-climbing is the biggest. But, once you learn the art of using the clutch and the shifter lever, a 125 is better at virtually everything else. It can be summed up easily. Would you rather ride a bike that feels powerful, or a bike that makes you feel powerful?

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