A Yamaha 125 is crazy reliable and cheap to maintain. A
piston sells for about $50. The worst possible engine failure
might result in the need for a cylinder, which sells for $350
A child can rebuild a 125—literally. And that’s not a bad
way to teach young riders how to work on motorcycle
The easiest and most common way to improve the output of the YZ is with an aftermarket pipe and silencer. Pro
Circuit and FMF have inventory on the shelf for around
$230 for a pipe and $130 for a silencer. You can pick up
as much as 2 horsepower on top, although you generally should rejet. Race fuel isn’t a bad idea, either. A Moto
Tassinari VForce3 reed is also good for a horsepower
bump and sells for $150.
Whether you should look at a 144 kit is debatable. The
horsepower gain is undeniable. You can gain substantial
low-end power and about 10 percent on top. But, there
are tradeoffs. A bored-out 144 has a piston that is 58mm—
4mm larger than stock, which results in more reciprocating
weight, less rpm on top and slower free rev. It can also
vibrate more and be harder to start. The bottom line is that
it will be faster, but not necessarily more fun.
The most common 144 kit is the Athena, which includes
all the parts you need including a cylinder. That’s good,
because you don’t have to modify any of your original
parts, and you can return to stock if you want. The kit sells
for around $850. If you damage your original cylinder, a
big-bore kit can be an affordable way to fix it. Millennium
Technologies can fix your cylinder, and about $200 will get
you a 58mm piston and gaskets.
THE NEW ROLE
Today, the 125 Yamaha isn’t a racer anymore. There are
a few organizations that have 125 classes, but most are
populated by bikes with oversized engines or KTM 150s.
Even a bored-out Yamaha can’t produce as much power
as the KTM 150, which is bored and stroked. That is actually somewhat liberating for the Yamaha. If you don’t race
it, you don’t need to ruin any of its appeal as a purely fun
bike, and you don’t need to spend money in a nearly fruitless endeavor. The bike offers dirt bike riding in its purest,
most fun concentration. It sells for $6290, and we challenge
you to find anything that provides as much return for the
buck. You won’t. ❏
Yamaha hasn’t changed the YZ125 motor in a very long time.
It will continue in Yamaha’s worldwide line for more time yet,
even if U.S. emission standards someday take it out of the
updated the YZ’s
styling in years.
You can have a more modern look with a
UFO plastic kit, which sells for $230.
Compared to a 250F, the YZ125 is 13 percent lighter
and has 20 percent less peak horsepower. A YZ weighs
201 pounds without fuel and produces about 34 horsepower on most Dynojet-style dynamometers. The bottom
line is that it can’t compete with a modern 250F on most
racecourses, but we already knew that from race results.
Compared to the KTM 125, the Yamaha has a little less
power everywhere. The Yamaha’s suspension, on the
other hand, is vastly superior. The debate over which is
better will rage on into the amoeba era.
The Yamaha YZ125 is a finished package. Yamaha has
had 10 years to get it right. The stock jetting is perfect,
unless you live in the mountains or have engine modifications. If you go to an aftermarket pipe, you might need to
swap the 430 main jet for a 440 or raise the needle one
THE YZ125! LONG LIVE