This Service Repair Manual covers engines, including models GH, WM, FH, 300400, Motorwheel, 6B, Z, and P, manufactured between 1919 and 1981. These robust 4-stroke, air-cooled engines, with displacements ranging from 6 to 23 cubic inches, were designed for reliability and efficiency, making them suitable for a wide array of applications. The manual provides comprehensive coverage of their engineering, ensuring owners and technicians have the resources to maintain these durable power units. This English-language manual offers detailed, step-by-step instructions and illustrations for comprehensive repair and service. It is designed to assist both DIY enthusiasts and experienced mechanics in performing routine maintenance, major repairs, and modifications. The content includes guidance on the ignition system, carburetor, governor, lubrication, electrical systems, and internal components, along with special instructions, special tools, and a conversion table, all aimed at facilitating inexpensive and effective maintenance.