2007 KTM 250SX

2007 KTM 250 SX: An All-New Generation of KTM Two-Stroke

The 2007 KTM 250 SX was a major redesign and one of the most important model years in the history of KTM's 250 two-stroke motocrosser. KTM did far more than revise jetting or suspension settings: the 250 SX received a new engine, frame, swingarm, bodywork, suspension package and ignition system. At a time when much of the motocross industry was concentrating almost entirely on four-strokes, KTM invested heavily in creating a new-generation 250 two-stroke.

The 249 cc engine retained the familiar 66.4 x 72.0 mm bore and stroke, but much of the hardware around those dimensions was redesigned. KTM developed a new cylinder and cylinder head, revised the exhaust-control system and reduced the weight of the top end. A redesigned crankshaft and flywheel increased rotational inertia, helping tame the abrupt power delivery that had characterised some earlier KTM 250s. The intention was not to make the 250 SX slow; it was to make its considerable horsepower easier to connect to the ground.

Intake changes were equally important. KTM continued with a Keihin PWK 36 S AG flat-slide carburettor but combined it with a revised V-Force reed-valve arrangement and updated intake tract. The result was improved throttle response and a smoother transition into the strong midrange. Owners dealing with worn or damaged original fuel-system components can compare alternatives through the MXOwners carburettor replacement guide.

Another useful feature was the selectable ignition mapping. The Kokusan digital CDI allowed the engine character to be altered between a more aggressive setting and a softer power delivery. Contemporary riders found the softer curve particularly useful on slippery or technical tracks because it helped the rear tyre find traction without removing the strong upper-rpm performance. This was a simple but effective way of making a powerful 250 two-stroke adaptable to different track conditions.

The chassis was completely redesigned around a new chrome-molybdenum steel frame and cast-aluminium swingarm. KTM had spent years addressing criticism that its motocross bikes did not turn as naturally as their Japanese rivals, and the 2007 generation represented a substantial step forward. The new frame was designed to improve lateral response while retaining the stability KTM riders expected. Revised bodywork also gave the rider more useful contact area for gripping the bike with the knees and moving forward during cornering.

Suspension changed significantly as well. The 250 SX received a 48 mm WP 4860 MXMA closed-cartridge fork with 300 mm of travel. Closed-cartridge construction helped maintain more consistent damping during long, rough motos and represented a major technical upgrade over KTM's earlier open-cartridge units. The rear retained KTM's distinctive linkless PDS concept, using a WP 5018 shock to provide 335 mm of rear-wheel travel. Owners servicing the front suspension can use the MXOwners fork oil reference, while the MXOwners rear suspension reference is useful when identifying or setting up the PDS system.

The five-speed transmission remained because the 250's broad power spread did not require six closely spaced gears. A hydraulic wet multi-disc clutch provided light and consistent operation, while the standard 13:48 final gearing suited motocross use. The engine used premixed fuel at a 1:40 ratio, and accurate oil measurement remains especially important for a high-output competition two-stroke. The MXOwners premix calculator can help when preparing different quantities of race fuel.

Historically, the 2007 KTM 250 SX showed exactly where KTM was heading. While rival manufacturers were beginning to abandon two-stroke motocross development, KTM produced an essentially new motorcycle and continued refining the format. The combination of a more controllable engine, selectable ignition maps, modern closed-cartridge suspension and a substantially improved chassis made the 2007 bike an important foundation for the KTM two-strokes that followed. It was not simply another version of the old 250 SX; it marked the beginning of a more modern generation that helped KTM become the manufacturer most strongly associated with keeping competitive two-stroke motocross alive.