This manual covers the Yamaha 4SD Outboard, a first-generation model produced in 1990. Designed as a reliable outboard motor, the 4SD represents a significant engineering achievement for its time, intended for a wide range of marine applications. Its robust construction and dependable performance are hallmarks of Yamaha's commitment to quality and innovation in marine propulsion. The purpose of this comprehensive English-language factory service manual is to provide detailed information for the maintenance, repair, and diagnostics of the Yamaha 4SD Outboard. It includes essential technical specifications, step-by-step repair procedures, exploded views, wiring diagrams, and troubleshooting guidance. This resource is invaluable for owners, technicians, and mechanics seeking to ensure the optimal performance and longevity of their 1990 Yamaha 4SD Outboard.