Honda C50 / C50M / C50MK1 / S50 Service Manual (P/N SM 50/65)

• Model coverage: Covers the Honda C50, C50M, C50MK1/C50(M)K1, C65, C65M, S50, and S65, followed by the factory CL70, C70, and C70M supplement.
• Engine repair: Details engine removal and installation, O.H.C. cylinder-head service, valve-seat inspection and repair, cylinder and piston checks, camshaft and cam-chain work, crankshaft inspection, and trochoid oil-pump service.
• Clutch and transmission work: Explains wet-clutch removal, inspection, adjustment, and reassembly; three-speed and four-speed transmission service; gear-shift mechanism inspection; and kick-starter repair.
• Chassis service: Addresses the handle and steering head, front fork and bottom-link front cushion, rear cushion, wheel bearings and axles, drum brakes, rear fork, drive chain, air cleaner, frame body, and muffler.
• Fuel and electrical diagnosis: Provides model-specific carburetor setting data, starting and charging tests, ignition checks, battery and selenium-rectifier service, and troubleshooting for lighting, turn signals, horn, and speedometer.
• Technical reference: Includes a numbered special-tool chart, component-specific torque and wear-limit tables, performance curves, dimensional drawings, fault charts, sectional and exploded views, and model-specific wiring diagrams.

This 156-page PDF contains five unnumbered front-matter pages, the numbered 50/65 shop-manual section on pages 1-121, and the CL70/C70/C70M supplement on pages 122-151. Honda Motor Co., Ltd.'s Service Publications Office describes it as a maintenance and servicing guide for servicemen, with system background also useful to sales staff. Because it proceeds into engine disassembly, precision measurement, and electrical testing, it is best suited to technicians and experienced owners who can work with pullers, gauges, and test equipment.

Complete System Architecture

The main engine section follows the air-cooled single-cylinder O.H.C. unit from its power-transmission layout and pressure-lubrication routes through internal inspection and reassembly. Coverage includes the cylinder head and valve train, valve-seat contact and guide work, camshaft timing, hydraulic/spring cam-chain tensioner, piston and rings, centrifugal filtration, right crankcase cover, crankcase breather, crankshaft, clutch, transmission, and kick starter. The drawings include the power-transmission diagram, trochoid oil-pump operating view, cam-chain tensioner detail, clutch oil-passage layout, transmission construction views, and carburetor cross-sections.

The model differences are not treated as interchangeable. C50/C50M/C65/C65M specifications cover automatic wet multi-disc centrifugal clutches and three-speed gearboxes, while S50/S65 use wet multi-disc clutches and four-speed transmissions. The 70-series supplement likewise separates the four-speed, manually clutched CL70 from the three-speed automatic C70 and C70M. It uses the CL70 as its base model and refers C70/C70M work back to the main 50/65 material where appropriate.

Fuel-System Coverage by Model

The carburetor setting table corrects an important fitment distinction: C50/C50M/C65/C65M use the listed 1000-series carburetor types, while S65 uses PW16FA6 or PW16FA10 and S50 uses PW16FA11. Factory data covers the throttle and venturi bores, main and air jets, bleed arrangements, needle and clip settings, throttle cutaway, slow-speed circuit, valve seat, pilot outlet, and fuel level. Separate procedures address high-, intermediate-, slow-speed, and idle adjustment, followed by fault tables for overflow, hard starting, poor idling, flat spots, weak acceleration, and loss of high-speed performance.

For CL70/C70/C70M, the supplement supplies a separate carburetor specification table but directs the reader to the 50/65 section for adjustment fundamentals. Its frame chapter similarly provides inspection specifications for the front and rear suspension, axles, brakes, and rear-fork bushings rather than repeating every base-manual procedure. The supplement identifies Tool Set No. 07900-1180000 as the shared C65/S65 tool kit.

Workshop Specifications and Diagnostics

Electrical coverage distinguishes the electric-start C50M/C65M circuits from the other 50/65 arrangements and explains generator performance, ignition-coil and condenser tests, spark advance, battery charging, and rectifier checks. Full wiring diagrams are provided for C50/C65, C50M/C65M, and S50/S65, followed by separate CL70, C70, C70M general-export, and C70M USA-type schematics. Troubleshooting tables extend beyond no-start complaints to engine noise, carburetion, steering pull or shimmy, clutch slip or drag, missed shifts and gear jump-out, suspension faults, brake performance, and drive-chain wear.

The special-tool chart identifies 21 factory tools and their numbers, including the valve-guide reamer 07984-0980000, clutch assembly tool 07960-0110000, flywheel puller 07933-0010000, and rear-cushion disassembly tool 07959-3290000. A valve-seat service note applies grinder 07782-0020000 (A set) from frame Nos. C70-D049302, C70-1062706, C70M-M051462, C70M-2010291, and CD70-116607 onward.

The manual warns that uneven cylinder-head tightening can crack or warp the head and cause sealing and compression loss. It also explains that operating the engine with the battery or electrical load disconnected can produce enough reverse current to puncture the selenium rectifier. Fuel handling, battery-electrolyte service, ignition testing, brake repair, and drive-chain work call for suitable workshop precautions and properly calibrated equipment.