TESTING THE TWO-STROKE CHAMP
THE NUTS AND BOLTS OF IT
Dave Simon bought the ’08 machine last year with 60
miles on it. He played with the machine off-road for a
bit and then got together with Emler and started testing
pipe combos. When Dave and Don learned about the
Two-Stroke National, they decided to go for it. They had
three weeks to get the bike ready to race motocross. They
started with a standard FMF Fatty and Shorty silencer.
Donnie has volumes of data on these machines, as he
helped David Vuillemin when he was with factory Yamaha.
They also tested some older works stuff that he built for
David back in the day, but they found that the evolution
of the Fatty actually outperformed the factory stuff from
2001. Muffler-wise, both said that the stocker was quite
good, but the FMF Shorty (with a ring in the back of it,
which is necessary for proper back pressure) helped coax
out some additional bark. In the end, Dave said that these
two mods produced the biggest gains in peak power and
Dave then modified the head, tightening up the squish
band and raising the compression. On the dyno, this
showed good gains pretty much across the board and,
interesting to Dave, a longer pull and gain in over-rev as
well. The duo decided to go with a Tassinari reed valve,
feeling it softened the hit ever so slightly yet added power
early on and pulled with a good hook on top. Overall,
it made things stronger, more tractable and ultimately
Donnie Emler and Dave Simon performed the top end surgery
on the ‘087 YZ250 cylinder. The head was modified, the porting soothed and expanded and this year they bored the carburetor for enhanced flow.
Dunlop’s new MX52 was the tire of choice for Glen Helen. It
proved to be very good on hardpack, plus hooked up nicely
in intermediate dirt. Regina chain, and a two-tooth smaller 48
tooth sprocket were used.
This year the FMF team added some serious power via more
porting and the carburetor work. This warranted a switch
to better fuel to fight detonation, Collier has the Moto Lab
people set the suspension up very plush to chew on the legendary hack at Glen Helen.