This manual covers the KX125 and KX250 motorcycles produced in 1990 and 1991. These models represent a significant evolution in the motocross line, built upon a strong heritage in racing. Engineered for performance, these 2-stroke, liquid-cooled machines feature advancements in suspension, engine output, and chassis design, making them competitive on the track. The KX series is renowned for its potent power delivery and agile handling, designed to meet the demands of motocross enthusiasts and racers. This comprehensive English-language service manual provides detailed guidance for the repair, maintenance, and troubleshooting of your 1990-1991 KX125 or KX250. It covers essential systems including the engine, drivetrain, chassis, and electrical components, with specific procedures for routine maintenance, engine rebuilds, carburetor adjustments, and power valve tuning. The manual also includes vital technical specifications, torque values, clearances, and wiring diagrams, equipping owners and technicians with the knowledge needed to keep these motorcycles in optimal condition.