Kawasaki KX125 / KX250 Service Manual (2003-2008)

Kawasaki developed its KX motocross family around lightweight competition chassis and powerful two-stroke engines, providing separate 125 cc and 250 cc machines for their respective racing classes. By the late 1990s, both motorcycles had adopted liquid cooling, reed-valve induction, the Kawasaki Integrated Power-valve System (KIPS), and steel perimeter-style frames. The 125 cc category emphasized momentum and close-ratio shifting, whereas the 250 cc category demanded broader torque and strong acceleration between corners. Riders chose between the smaller machine's six-speed gearbox and the larger machine's five-speed unit, but each relied on a responsive carbureted single-cylinder engine. As four-strokes began challenging traditional motocross classes, Kawasaki undertook a major overhaul of both two-stroke models to remain competitive during the early 2000s.

For the 2003 model year, Kawasaki substantially redesigned the KX125 KX250 range. Both M1-generation motorcycles received newly designed steel perimeter frames, refreshed bodywork and suspension geometry intended to improve cornering precision without sacrificing stability. Kawasaki's June 2002 factory announcement of the 2003 KX models detailed these developments, including a revised five-speed KX250 shift mechanism and a six-speed KX125 arrangement. The smaller KX125-M1 adopted a redesigned 124 cc engine with a 54.0 mm bore and 54.5 mm stroke and a 38 mm Mikuni TMX carburetor. The KX250-M1 used a 249 cc engine with a 66.4 mm bore and 72.0 mm stroke, a Keihin PWK38S carburetor and five-speed transmission. Both engines featured improved cylinder coatings and revised KIPS exhaust-valve operation.

The 2004 KX125-M2 and KX250-M2 continued the M-series developments but incorporated distinct engine and chassis refinements. In the KX250, Kawasaki revised port timing, cylinder compression, induction and exhaust components in pursuit of improved response across the rev range. The 2004 KX250 was not simply a cosmetic continuation of the 2003 motorcycle; its tuning changes sought stronger output and smoother power delivery in different track conditions. MotorcycleDaily's contemporary 2004 review records these changes. The KX125 also gained revised compression specifications and changes to its frame geometry, ergonomics and rear-suspension linkage. Through 2004, both models retained carburetors, kickstarters, hand-operated wet clutches, hydraulic disc brakes and the typical competition-oriented maintenance demands of high-output two-strokes.

The two product lines followed different documentation and development paths after 2004. The KX250 received a substantial 2005 revision, featuring a different KIPS operating strategy, revised cooling passages, changed steering geometry and twin-chamber Kayaba forks. Kawasaki therefore did not treat the later 2005–2008 KX250 models as the same M1/M2 series documented for 2003–2004. By contrast, the KX125 continued through the M3 in 2005, M6F in 2006, M7F in 2007 and M8F in 2008 in the markets reflected in Kawasaki's model identification records. Refinements across those KX125 versions included changes to riding position, controls, ignition specifications and chassis parts. A contemporary 2005 Kawasaki model report records the adjustments to both motorcycles, while highlighting the greater scope of the KX250 redesign.

The early-2000s two-stroke motocross generation also developed alongside a major shift toward four-stroke racing motorcycles. In August 2003 Kawasaki announced the new 250 cc four-stroke KX250F, which joined rather than replaced the established two-stroke KX250 identity. Those motorcycles are mechanically different despite sharing the KX and 250 designations: the carbureted two-stroke KX250 used different engine internals, maintenance procedures and gearing from the four-stroke KX250F. Kawasaki's original KX250F announcement documents that transition. The KX125's continued development through its 2008 M8F designation illustrates the survival of the smaller two-stroke motocross platform in selected markets. The distinction between 2003–2008 KX125 and 2003–2004 KX250 variants is essential to the history of these machines. Related titles appear in the Kawasaki service manual catalog.