This workshop service and repair manual covers Mercury Outboard models manufactured between 1966 and 1989, specifically addressing pre-1990 models. These outboards are known for their robust engineering and reliability, offering a range of configurations from 90HP to 300HP. The manual encompasses various engine types, all water-cooled, and features a lower unit or gearcase transmission. This comprehensive guide is designed to provide in-depth information on these powerful and enduring marine propulsion systems. The purpose of this extensive manual is to serve as the definitive resource for maintaining and repairing your Mercury Outboard. It details essential maintenance procedures, including lubrication and fuel system requirements, alongside in-depth troubleshooting and diagnostic protocols. Technical specifications, such as torque values, fluid capacities, and electrical system details, are thoroughly documented. This English-language manual is an indispensable tool for owners, mechanics, and technicians seeking to ensure the optimal performance and longevity of their Mercury Outboard engines.