1985–1988 Yamaha YFM80N Service Manual (P/N LIT-11616-04–84)

• Official Yamaha factory publication P/N LIT-11616-04-84, 1st edition September 1984, 3rd printing April 1986
• 179-page PDF for the YFM80N, factory model code 55X, with vehicle starting number *JYA55X00FC000101 and engine starting number 55X-000101**
• Factory specifications identify a 79 cm³ 4-stroke SOHC single-cylinder engine, VM16SH Mikuni carburetor, wet centrifugal automatic clutch, constant-mesh 4-speed transmission, and shaft drive
• Includes maintenance specifications, torque data, lubrication and cable-routing diagrams, electrical schematics, and dedicated starting, charging, and ignition troubleshooting

Powertrain and Chassis Systems

This factory service manual breaks down the Yamaha YFM80N powertrain, chassis, and electrical systems for routine maintenance and complete repair work. Machine identification comes first, with the factory model code and starting vehicle and engine numbers needed to confirm the correct application before parts or procedures are selected. The publication does not state a 1985–1988 model-year range or identify a later serial break, so buyers should match their machine to the 55X code and listed starting numbers rather than relying on model year alone.

Engine coverage goes well beyond basic tune-up work. The repair sections cover engine removal and remounting, cylinder head and cylinder disassembly, camshaft and cam-chain components, rocker arms, valves and guides, piston and rings, clutch, oil pump, crankshaft, transmission, middle gear, CDI magneto, and crankcase separation. Factory procedures also cover valve-guide replacement, valve-seat cutting and lapping, cylinder and piston measurement, clutch inspection, transmission and shift-component inspection, and middle-gear shim selection.

Several limits make the manual useful when deciding whether parts can be reused. Cold valve clearance is 0.05–0.10 mm intake and 0.075–0.125 mm exhaust. Standard cylinder bore is 47.000–47.005 mm with a 47.1 mm wear limit. Sea-level compression pressure is specified at 1,177 kPa / 171 psi, with 981 kPa / 142 psi minimum and 1,275 kPa / 185 psi maximum. Periodic engine-oil capacity is 0.85 L, total engine capacity is 1.0 L, and the final gear case takes 0.1 L of the specified grease.

The chassis chapters cover front and rear wheels, rear drum brake, steering shaft, steering knuckles and tie-rod ends, rear axle, and shaft-drive components. Factory toe-in is 0–3 mm, with separate inspection limits for wheel runout, rear-axle runout, brake lining wear, bearings, and steering parts. Torque callouts are provided throughout the repair procedures, including 40 N·m for the flywheel nut, 60 N·m for the clutch boss locknut, and 65 N·m for the front axle nut.

Carburetion and Electrical Diagnosis

The VM16SH Mikuni carburetor receives its own removal, disassembly, inspection, assembly, installation, and adjustment section. Factory calibration data includes a #80 main jet, 3HP9 jet needle, #12.5 pilot jet, 3.5 ± 1.0 mm fuel level, 21.0 ± 0.5 mm float height, and 1,800 ± 50 r/min idle speed. This is the type of data needed for actual carburetor repair rather than generic cleaning instructions.

Electrical coverage includes the YFM80N master wiring diagram plus separate starting-system, charging-system, and ignition-system circuit diagrams and troubleshooting charts. Starter motor, starter relay, starting circuit cut-off relay, charging output, ignition spark gap, ignition coil, pick-up coil, and source coil are addressed with test specifications. Reference values include 1.6 Ω ±10% primary ignition-coil resistance, 6.6 kΩ ±20% secondary resistance, 330 Ω ±10% pick-up coil resistance, and 380 Ω ±10% source coil resistance. The ignition spark-gap test specifies a 6 mm minimum gap.

The manual also identifies the factory tools used for these jobs, including the YU-33223 Compression Gauge, YM-01312 Fuel Level Gauge, YM-91042 Clutch Holder, YU-01135 Crankcase Separating Tool, YU-33975 Ring Nut Wrench, and YU-03112 Pocket Tester. Their inclusion is useful when planning a repair that requires more than normal hand tools.

Diagrams, Procedures, and Repair Details

Representative technical views include the carburetor sectional view, crankshaft assembly, transmission assembly, rear brake exploded view, shaft-drive assembly, and YFM80N wiring diagram. The appendices add general and maintenance specifications, general torque specifications, conversion tables, lubrication diagrams, and cable-routing drawings.

Among the factory procedures are jobs many owners would otherwise send to a repair shop: engine removal, valve and valve-seat service, crankcase separation, clutch overhaul, transmission inspection, middle-gear and shaft-drive work, carburetor fuel-level adjustment, steering and toe-in adjustment, rear-brake service, starter testing, and electrical fault isolation.

There are also manualspecific assembly traps worth knowing about before opening the machine. During crankcase separation, Yamaha specifically warns against striking the crankshaft or crankcase mating surfaces because those parts can be damaged. When replacing a wheel bearing, force is to be applied through the outer race rather than the center race or balls to avoid damaging the bearing. In the shaft-drive U-joint assembly, the needle rollers can fall out of position during bearing installation; if that happens, the yoke will not seat correctly and forcing it can damage the bearing assembly.

ATV repair in this manual includes rotating engine parts, battery and ignition circuits, brake components, and work that requires the machine to be securely supported. Use appropriate protective equipment and proper support before service, and have safety-critical brake, steering, or electrical work checked by a qualified technician when necessary.