This workshop service repair manual covers Mercury Mariner 30JET HP / 40 HP outboard engines. These engines are engineered for reliability and performance, featuring a 4-cylinder outboard configuration and available in 30 HP and 40 HP variants. The manual provides comprehensive information for maintaining and repairing these units, encompassing essential systems such as fuel, ignition, electrical charging, and the oil injection system. It details service procedures for key components including the powerhead, mid-section, lower unit, and jet drive, along with various tilt and trim designs and the rewind starter system. This extensive English-language manual is designed for both do-it-yourself enthusiasts and experienced mechanics. It offers hundreds of pages of detailed, step-by-step instructions, accompanied by numerous photos and illustrations to clarify complex procedures. The manual includes troubleshooting guides, particularly for electrical systems with integrated wiring diagrams, and covers timing, synchronization, and adjustment procedures. Its purpose is to equip users with the knowledge necessary to effectively maintain, diagnose, and repair their Mercury Mariner outboard engines, ensuring optimal performance and longevity.