Thus, when you see a KTM 300SX in the pits, you are
looking at a labor of love, and, in most cases, a dream
fulfilled. Installing the 300cc top-end kit to the KTM
250SX is no more difficult than doing a top-end job on
any two-stroke. You install the new cylinder, head, piston,
rings, power-valve mechanism, gaskets and black box and
you are ready to roll. And, on the plus side of the ledger,
your old KTM 250SX top end remains intact and can be
put back on without any drama when you sell the bike.
WITH OUR SETUP, WE GAINED 2. 7
HORSEPOWER AT 6000 RPM, 5. 5
HORSEPOWER AT 7000 RPM, 2. 4
HORSEPOWER AT 8000 RPM AND 2. 2
HORSEPOWER AT PEAK.
But all KTM 300SX motocross projects are not the
same. On MXA’s 300SX, we elected to run a Pro Circuit
300XC pipe, Pro Circuit KTM 250SX shorty R-304 silencer
and a 9-ounce Steahly flywheel weight. For jetting, we
ran the stock main and needle but went to a 40 pilot jet
(from a 42) and dropped the needle one clip position. For
fuel, we use 91-octane pump gas with a splash of VP
C- 12 to ensure fuel quality. With our setup, we gained 2. 7
horsepower at 6000 rpm, 5. 5 horsepower at 7000 rpm, 2. 4
horsepower at 8000 rpm and 2. 2 horsepower at peak over
our stock KTM 250SX engine. Could you ask for more?
The head was milled to compensate for raising the cylinder to match the transfer ports. The compression ratio was altered to allow
91-octane pump fuel to be used.
MX1 Suspension fit their Positive Seal mid-valve system in
the fork to help hold it higher in its stroke. This allowed the
compression damping to be softer to reduce mid-stroke
harshness. The fork tubes were treated with a DLC coating.