I’ve long had a love affair with the Yamaha YZ125. That adulation is shared by the rest of the MXA wrecking crew. Need proof? The YZ125 won our
“ 125 Two-Stroke Shootout” this year, coming out ahead
of bikes that are newer and faster. On paper, it would
appear that the aged 2015 YZ125 is cannon fodder;
instead, it’s a winner, thanks to a broad powerband,
lightweight chassis and the best stock suspension
known to man. The 2015 Yamaha YZ125 is the ultimate
example of refinement—the Chateau Margaux of the
Given the choice of any two-stroke on the MXA lot,
I jumped at the opportunity to build my vision of the
dream Yamaha YZ125. And despite my best intentions
to keep the project bike within the bounds of reason, I
melted the company credit card. No sane person would
spend over $6390—the retail price of a 2015 YZ125—
on aftermarket parts alone. Yet, that’s how much my
changes cost, and I came away with a bike revered by
Here is how I did it.
(1) Engine. Power is the name of the game in the
125cc two-stroke class. In most club-race organizations,
it’s perfectly legal to run upwards of 150cc in the 125
class. I pondered installing an Athena YZ144 kit, but
I didn’t feel that the juice was worth the squeeze.
Upping the displacement meant splitting the cases and
grinding away material to account for the oversized
piston. There were easier ways to gain 2 horsepower.
First, I replaced the stock reed block with a Boyesen
RAD valve. The stout carbon fiber reeds and aerodynamic reed-block shape optimize airflow for better
throttle response and peak horsepower. The RAD
valve bolted on without any modifications. Good stuff.
Second, while I had the carburetor off, I installed
a Boyesen Power X-Wing. It essentially streamlines
air being sucked through the airboot and directs air
more efficiently into the carburetor. Daniel Bernoulli
would highly endorse the Power X-Wing, but since the
physicist died more than 200 years ago, I’ll praise the
Boyesen invention for him. Third, I ditched the stock
plastic filter cage for a Twin Air Powerflow kit. The
sturdy aluminum cage ensures that the Twin Air foam
air filter mounts properly and doesn’t create leaks.
It’s peace of mind. Fourth, I rang up Pro Circuit and
ordered a Works pipe and Ti- 2 titanium/Kevlar silencer.
Paying $329.95 for the silencer alone seemed ludicrous,
but it knocked about 2 pounds off. Decreasing weight
wasn’t my overall goal for a bike that rolls off the
showroom floor at 199 pounds. Still, improving power
while trimming the fat is a win-win.
( 2) Forks and shock. MXA has access to the most
expensive suspension hop-ups that money can buy. I
fought the urge to go shopping for Showa A-kit forks
or suspension work. Why? Nothing beats the stock
Yamaha YZ125 forks and shock. I didn’t want a tuner
to stiffen up the suspension for my 175-pound carcass,
despite the fact that Yamaha valved the Kayaba SSS
units for a 130-pound kid. There’s no reason to spend
a dime on the stock suspension. It’s that good. I have
never ridden with suspension that works so well across
such a wide range of skill levels and weight variances.
( 3) Triple clamps. Ride Engineering recently
introduced a 22mm-offset triple clamp for the YZ125