The powerplant of the Rock River YZ250 was massaged by
C4MX. Bolt-on power came from FMF, Boyesen and K&N.
The XTrig clamps have a slot cut in them for more equal
clamping surface in order to not disrupt the internals.
The more power you have, the more brakes are needed.
Galfer came to the rescue with a 270mm front rotor.
What better way to groom a rider than to give him
a chance to race on a true-to-life AMA National track—
not on a weekday and not at the amateur race the day
before, but right in the middle a full-blown AMA 250/450
National. The problem is that the AMA only allows
licensed AMA Pros to enter an AMA National. Timmy
Badour wasn’t ready to go up against the might of the
factory teams or the clout of the AMA rule book, so
what did Rock River Yamaha do? They signed him up
to ride the special two-stroke exhibition race at the Glen
Helen National. Racing in the FMF-sponsored Two-Stroke
Challenge meant that Timmy would race on the same
AMA National track as the heroes, in front of a massive
crowd of fans, and be the first race of the day. The
problem? Timmy didn’t have a two-stroke. That is where
Rock River stepped in to make it happen.
The Rock River YZ250 project started with a brand-spanking-new 2015 Yamaha YZ250. It came right off the
showroom floor of Rock River Motorsports in Johnson
Creek, Wisconsin. The Rock River team put together a
laundry list of team sponsors that anted up for the build,
along with some other companies that wanted to support
the cause. By the end of the day, Timmy had raced on
one of the most challenging courses on the AMA circuit
and come away with a fourth-place finish. Timmy got
some good experience, and next year he hopes to be on
the starting line for the AMA 250 National class.
With Timmy’s work done, it was time for MXA to step
in and take the reins of the Rock River Yamaha YZ250.
In stock trim, the YZ250 is hard to beat. We usually just
bolt on an aftermarket pipe and oversized front brake
rotor to make the totally refined, if a little dated, YZ250
race-ready. The Rock River team went a few steps further
to accommodate the massive hills and competition at the
Glen Helen Two-Stroke Challenge.
ENGINE: The YZ250 engine was sent over to C4MX
for a facelift. C4MX did not go whole hog on the engine,
just a few simple tricks to make it purr. An extra base
gasket was added to fine-tune the deck height of the
high-compression Vertex piston to match the head
perfectly, which had been machined off by 30 thousandths. A Boyesen Rad Valve, K&N filter and FMF
Factory Fatty pipe were installed for bolt-on power.
Through its veins flowed the great-smelling power
of VP Racing C- 12 fuel (a must-have for a high-compression two-stroke setup).
What can we say? The MXA test riders loved this
engine. It had improved power across the board and was
easy to ride. We liked that the clutch did not have to be
abused, as the bottom-end power was snappy out of
corners. Over-rev was increased, which extended our
shift points and made the engine hit harder when shifted
SUSPENSION: Enzo is a big help to the Rock River
Yamaha AMA National team, so it was only natural that
they would work on Badour’s YZ250. Enzo re-valved
and resprung the suspension front and rear. They also
installed their proprietary spring tubes in the forks, which
replaced the stock spring perch. The rear shock got an
Enzo high-speed adjuster.
Although the suspension was set for a welterweight
at Pro level, we were still impressed with how the
suspension worked for our heavier test riders. Both the
front and rear suspension had a fluid progression, with
enough bottoming resistance to allow for Glen Helen’s
big hits and infamous rock walls. The slow-moving
suspension was confidence-building, as the MXA test
riders could react to what the bike was going to do. In
stock trim, the YZ250’s front end is great for all levels,
except an AMA National track on race day.