This manual covers the GS250, GSX250, GS400, and GS450 Twins, produced between 1979 and 1985. These parallel-twin motorcycles, known for their reliability and performance, were manufactured in Japan. They represent a significant part of heritage in the motorcycle market, offering a blend of engineering quality and user-friendly operation. The models feature air-cooled, 4-stroke parallel-twin engines with varying displacements from 247cc to 448cc, and typically utilize a 5-speed transmission and a wet, multi-plate clutch. Common configurations include a front disc brake and a rear drum brake, along with essential lighting systems. This comprehensive service repair workshop manual provides factory-issued, technician-level detail for all models and repairs, covering all systems from A-Z. It includes step-by-step instructions, detailed illustrations, exploded views, and complete wiring schematics for the engine, fuel system, ignition, transmission, brakes, electrical system, and chassis components. The manual is designed to assist GS series twin motorcycle owners and independent mechanics in performing routine maintenance, troubleshooting, and in-depth repairs, ultimately empowering them to maintain and optimize their vehicles.