This manual covers the GPZ400 and GPZ550 models from the first generation of the GPz series, produced between 1979 and 1985. These motorcycles are known for their inline-four engines, featuring DOHC and four valves per cylinder, air-cooled for efficient operation. The GPZ400 typically offers around 398cc displacement and approximately 46-50 HP, while the GPZ550 boasts approximately 553cc and 58-65 HP. Both models utilize a 6-speed constant mesh transmission and a wet multi-plate clutch. They are built upon a steel tube frame and feature signature Uni-Trak rear suspension, coupled with telescopic front forks and front disc brakes for reliable performance and handling. This comprehensive workshop and service repair manual provides detailed information for owners, mechanics, and service professionals. It covers essential areas including routine maintenance, periodic checks, and lubrication, as well as in-depth troubleshooting and repair procedures for all major systems. Key sections include engine specifications, fuel system servicing (including carburetor tuning), cooling system maintenance, transmission and drive system repairs, electrical system diagnostics with wiring schematics, chassis and suspension adjustments, and bodywork procedures. The manual is designed to facilitate everything from basic upkeep to complete engine rebuilds and electrical system diagnostics, ensuring the longevity and optimal performance of your GPZ motorcycle.