This is the 2010 KTM 250SX the way the MXA test crew raced it. As a rule, we don’t go for exotic or expensive mods, but if push comes to shove, we will seek competent help (no matter how much it costs). Use MXA’s mods as a guide.
Handlebars. Taller MXA test riders felt that the handlebars were too low. We opted for the economical choice and got taller bar clamps from KTM’s parts catalog. These were about $25, versus handlebars at about $90.
|Forks. Unlike KTM’s SXF four-stroke models, the KTM 250SX gets the
European-spec spring rates. As such, the stock 0.44 kg/mm fork springs
are way too soft for American riders. Swap out the stock 4.4 springs for
stiffer 0.46 springs and, if you are fast or fat, go to 0.48 springs Use the oil height to adjust the feel through the mid-stroke.
||Shock. The stock shock spring is a 6.9 kg/mm Euro-spec spring rate.
If you are in the 150- to 185-pound range, you will be happy with this
spring, but heavier or faster riders will need to go to a 7.2 kg/mm
spring. We run the sag at 110mm, the low-compression on 19 clicks out,
high-speed compression at one turn out, and the rebound on 15 clicks
Gearing. The stock KTM 250SX gearing is 13/48, but every MXA test rider runs 13/49 to tighten up the ratios.
||Silencer. We drop-kick the stock silencer and opt instead for a Pro
Circuit or FMF silencer. They are lighter and improve the throttle
response. Even if you stick with the stock exhaust pipe, the silencer is a good idea.
||Power valve springs. When you buy a KTM 250SX, you get two additional
power valve springs: one red (softer) and one green (stiffer). Changing
the power valve spring affects the final rpm settings by about 500 rpm.
With the red spring, the power valve is fully open at 7000 rpm. With the
stock yellow spring, the power valve is fully open at 7500 rpm. The
green spring will allow the power valve to be fully open at 7900 rpm. We
run the red spring. The preload on the power valve spring can also be
adjusted. It affects the initial movement of the power valve’s opening.
The standard setting (from the factory) is set so the power valve starts
to open at 5500 rpm.
Pipe mounts. KTM 250SX owners need to pay special attention to the pipe mounts and the two pipe O-rings. The O-rings wear out quickly and should be replaced at regular intervals. You can tell that they need to be replaced when the pipe starts to vibrate. If the pipe mount bolts loosen up, the brackets will crack. The Pro Circuit pipe is the one that Jeremy McGrath developed during his short stay at KTM—Pro Circuit still offers it in several forms.
Jetting. The stock jetting is good. We run the stock 158 main, NIEI
needle (in the third clip), 42 pilot, and the air screw one turn out
most of the year. But, in the heat of summer, we switch to a 40 pilot and
turn the air screw out 1-1/2 turns.
There are two ignition curves in the 250SX black box. You can switch
back and forth by unplugging the connector. In our opinion, unless you
want the bike to feel slower, leave it plugged in on the aggressive
Gas cap. We use dykes to cut the locking tabs off KTM’s push-button gas cap so that it works without us having to use two hands. If you have a pre-2010 KTM 250SX, swap the stock 1.4 kg/cm2 radiator cap for a 1.8 cap (this is what the 2010 models use).
Side panels. To enable test riders to lift the 250SX onto a bike stand, we use a box cutter to make a curved cutout in the right side panel.
KTM Motorcycle tests