TESTING THE TWO-STROKE CHAMP
AND IN THE END
FMF, Dave Simon and Sean Collier won their second
MTA Two-Stroke National on the same 2008 Yamaha
YZ250, thanks to a mechanical suffered by leader Justin
Jones. Since the win, Dave has shifted the YZ back into
Arizona dual-sport mode and rides it to and from work.
That’s not too bad for a four-year-old ring ding that has
been on the short end of the pool cue when it comes to
manufacturing updates. It shows you that good old-fash-
ioned hot-rodding still holds water in the dirt bike-racing
world and lets you stay competitive in a market domi-
nated by evolution. The Yamaha YZ250 is a darned good
platform for building a racer. o
more competitive. With the head mods, they had to up the
octane rating to avoid detonation. A 50/50 AV gas/Premium
did the job. Silkolene oil at 40:1 was the lube of choice.
Moto Lab Dirt Bikes handled the suspension. They put
in new, smoother fork valving through the mid stroke, and
of course a meatier full-compression load for the big hits.
It was valved and sprung specifically for Sean Collier’s
height and weight. Shock-wise, the goals were similar.
They wanted smoother transitions in chop for better traction. They bolted on a Fastway steering damper, which
really helped relieve the twitchiness, Pro Taper handlebars,
ODI grips, and a Works Connection AOF clutch perch
assembly. A 20mm-longer Hammerhead shifter was used,
along with a Torc1 folding-tip brake pedal.
And with that combination, FMF and Sean Collier won
the 2013 Two-Stroke Nationals.
For 2014, they didn’t reinvent the wheel; they just made
it faster. Donnie wanted more power, so they added some
additional work to the transfer ports (nothing dramatic),
and then went to work on the carburetor. They bored it
larger (in increments), testing and dynoing each cut in an
effort to avoid hurting bottom power too much. In the end
they got a huge boost, starting at 3000 rpm, and it got
the horsepower numbers from last year’s 49 all the way to
52. 3. According to Donnie, the backside number (as the
power starts to hit the wall and drop dramatically) was six
horsepower up from stock. It was a ripper.
Now that they had the power there, it was actually a bit
too much, even under the guidance of a pro rider. Even
though they didn’t add anything to the static compression
of the engine, the larger carb added a lot to the running
compression of the engine, requiring a new fuel mix of half
VP brand C- 12 and half AV gas for an octane of about 106,
just to keep detonation under control. The bottom-end hit
became so abrupt that they added an 11-ounce flywheel
weight from Steahly to keep the wheel from lighting up and
to improve traction. The stock 50-tooth rear sprocket went
down to a 49, but Sean wanted a 48 since he felt peaked
out with the 49 on the track.
Different clutch plates were used to try to harness
the power. After the plates started to come apart at the
end of the first moto, sending a first-place Sean back to
third, it was apparent they should have stayed with stock
fibers. They band-aided the clutch for the second moto,
but it was still slipping, which made for getting pulled on
the start and some struggling throughout the race.
The final modifications were to the brakes, which were
updated to a larger Galfer front rotor kit. Dead End vibration inserts from Best Dual Sport Bikes were used to kill
the two-stroke’s vibration, making the race and ride much
more enjoyable for Sean’s arms and hands.
An FMF Shorty
muffler is super
light (it uses titanium) and helps
with very focused
the ring in the
back of the core.
is 20mm longer
A Works Connection AOF perch helped with clutch pull while
Best Dual Sport Bike Dead Ends reduced vibration a ton.
The FMF Fatty has evolved mainly due to David Vuillemin’s
time racing for Factory Yamaha. Donnie actually built a one
off special using a Vuillemin spec and the production Fatty
made better power!