This workshop manual provides comprehensive coverage for various 4-cycle engines manufactured between 1919 and 1981, including pre-1981 models. These air-cooled engines, ranging from 6 to 23 cubic inches in displacement, are known for their robust engineering and reliability across numerous applications. Specific model codes covered include GH, WM, FH, 300400, Motorwheel, 6B, Z, and P. The manual is designed to assist users in understanding and maintaining these durable power units. This detailed English-language service repair manual offers step-by-step instructions and detailed illustrations and diagrams for a wide array of engine systems and components. It covers essential areas such as the ignition, air cleaner, carburetor, governor, compression, oil, and electrical systems, as well as internal parts like piston rings, connecting rods, crankshafts, and camshaft gears. Additionally, it includes information on mechanical rewind starters, alternators, and generator starters. This manual is an invaluable resource for do-it-yourself enthusiasts and experienced mechanics alike, providing an inexpensive method to ensure these engines remain in optimal working condition.