This workshop service manual covers the Mercury Mariner 30 Marathon, a 2-stroke, water-cooled outboard engine with a 427 cc displacement and 30 horsepower output. Featuring a carburetor fuel system and CDI ignition, this engine is designed for reliable performance in demanding marine applications. Its robust engineering ensures durability and efficiency, making it a dependable choice for various boating needs. This comprehensive manual provides essential information for maintaining and repairing your Mercury Mariner 30 Marathon. It details critical maintenance procedures, including lubrication requirements such as the recommended 2-stroke outboard oil and a 50:1 fuel-oil mixture ratio, along with intervals for spark plug replacement, water pump impeller service, and lower unit oil changes. Technical specifications, troubleshooting guidance, and safety information are also included to assist owners and technicians in ensuring optimal engine operation and longevity.