Combo: The only thing holding back the stock KTM 250SX is the cranky WP suspension, but once Fox Factory was done massaging them, our 250SX could go anywhere and do anything.

There can be no doubt that the MXA wrecking crew loves the KTM 250SX two-stroke—and we think that every two-stroke fan should embrace what this bike represents. KTM’s engineers could have sat on their hands and let the marketing men determine what the Austrian factory should build and sell, but they believed in two-strokes. And because the KTM marketing men listened to the engineers, KTM has proven beyond their wildest dreams that the offroad motorcycle market is willing to seek out the best two-stroke technology.

The marriage of KTM’s engineering prowess and its management’s willingness to invest dollars into R&D has not just resulted in the best 250cc two-stroke ever made, but two-stroke sales figures that stunned their Japanese competitors. It is reported that KTM sells more two-strokes dirt bikes than most other brands sell four-strokes. That is the kind of success a manufacturer deserves when it goes out on a limb to build bikes that its competitors have declared null and void.

Bulging muscles: FMF’s Factory Fatty pipe looks great on the KTM 250SX. Since the engine is built in-house by FMF, this pipe was finely tuned to the power characteristics. Get yourself a Factory Fatty for $249.99.

KTM’s two-stroke program is the gem of the industry. It includes 50cc, 65cc, 85cc, 125cc, 150cc, 200cc, 250cc and 300cc displacements in both motocross and enduro versions. And these aren’t leftover engines that haven’t seen a new blueprint in a decade. Nope! KTM has spent serious R&D dollars on its two-stroke engines, pioneered new categories and constantly refined its effort.

Yet, as much as the MXA wrecking crew loves the KTM 250SX two-stroke’s charms, we aren’t blind to its faults. We aren’t geniuses, but we know what every hardcore KTM 250SX owner knows—things could be better. And that is where FMF and Fox Factory come into play.


As friends of three-time AMA National Champion Kent Howerton, the MXA gang takes an interest in the riding skills of Kent’s son, Derek. Derek has become a YouTube sensation with his radical scrubs and high-rpm videos on his personal KTM 250SX. With a few calls to Derek’s sponsor, Fox Factory, we made arrangements to have an exact replica of Derek’s bike built for us to test.

One-off: FMF doesn’t do engine work for customers anymore, but that doesn’t mean they’ve forgotten how. Every once in a while, Donny Emler gets an itch he needs to scratch. Donny milled the head and reshaped the combustion chamber for a solid power increase.
Dialing for dollars: The Fox Podium RC3 shock offered a lot of adjustment compared to the OEM WP shock. The MXA test crew went lighter on both low- and high-speed compression damping than what Derek Howerton runs.

MXA’s FMF/Fox Factory project had two simple goals:

(1) Fix the KTM 250SX’s weak link—the suspension. Fox Factory, inventors of the most successful air shock in motocross history, was up to the task. They took off the stock WP rear shock and replaced it with a Fox Podium RC3 rear shock, valved to Derek Howerton’s spec. The shock used a 5.4 kg/mm shock spring and featured all of Fox Factory’s inventive technology, including their proprietary Bottom Out Control (BOC) clicker, Anti Wallow Control (AWC) and FAST spring preload adjuster. As a bonus, with the Fox FAST system, we didn’t have to deal with KTM’s terrible nylon preload ring. The Fox Factory RC3 shock retails for $995.

The WP forks also got the Fox treatment with new valving, damping and tool-free, 35-click compression adjuster caps. The forks used the stock KTM 0.44 kg/mm springs. The fork valving retails for $225, and the compression adjuster caps cost $199.

Cowboy way: Fox dialed in Derek Howerton’s exact fork settings, but we ended up lightening up the compression and rebound. The front end rode a bit high for our tastes, so we slid the fork legs up 5mm.
Fling: On a powerful 250 two-stroke you feel the motion thanks to the instantaneous vibes of the engine. Even if it’s not faster than 450 four-stroke—it feels faster.

(2) While Fox Factory worked on the suspension, FMF took over the engine development program. The project was quite unusual for FMF, because the only product that they sell on the Derek Howerton replica is the FMF exhaust system and silencer. The ported cylinder, milled head, bored carburetor (from 36mm to 39mm) and jetting were all handled by FMF, even though you can’t order them from Uncle Donny.

In the beginning, we struggled with the setup of both the suspension and the 39mm jetting. But, with basic tuning techniques, we resolved every issue and got the bike working like a dream. In back-to-back tests against our stock KTM 250SX, there was no comparison. The powerband was broader. It hit harder and pulled farther.


We originally ran VP Racing C-12 in the engine, but over time we learned more about the engine’s needs and worked our way down to a 50/50 mix of pump gas and C-12.

As for the suspension, it was lightyears better than the stock WP stuff. After riding the Fox Factory-equipped KTM 250SX, the MXA test riders didn’t ever want to ride the stock WP suspension again.

Brass ring: With the increase in compression, the need to run race gas and the bored-out 39mm carb, the jetting was way off. We went to a larger 168 main, stock pilot and clip in the middle.

For more information about Fox Factory suspension mods, go to www.ridefox.comor call (800) 369-7469. FMF Racing can be reached at www.fmfracing.com or (310) 631-4363.


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