This workshop service repair manual covers the Mercury Mariner 30 Marathon outboard motor. This 2-stroke, 30 HP, water-cooled engine features a CDI ignition system and is carbureted for fuel delivery. It is designed for pre-mix oil lubrication, typically at a 50:1 ratio. The transmission offers a 2.15:1 gear ratio with forward, neutral, and reverse positions. Available in standard or counter-rotation, this motor supports various propeller pitches and includes options for electric start and manual start, along with manual tilt and power trim systems. This comprehensive English-language manual provides detailed service, maintenance, and troubleshooting information. It covers essential procedures such as spark plug replacement every 100 hours or annually, and gearcase oil changes at the same interval, with a capacity of approximately 200-250 ml. Diagnostic guidance is included for common issues like engine starting problems, performance degradation, and electrical faults. This resource is designed to assist owners, mechanics, and technicians in effectively maintaining and repairing their Mercury Mariner 30 Marathon outboard motor, ensuring optimal performance and longevity.