1987-1990 Yamaha Jog Service Manual

The Yamaha Jog is a popular 50cc two-stroke scooter produced from 1987 to 1990, featuring automatic transmission, oil injection system, and lightweight design. This service manual covers maintenance procedures, troubleshooting, and repair instructions for all models within this production period. Below are key sections for specifications, maintenance schedules, engine service, electrical systems, and troubleshooting.

1. General Specifications 2. Maintenance Schedule 3. Engine Service 4. Fuel System 5. Electrical System 6. Transmission 7. Brake System 8. Suspension 9. Troubleshooting 10. Wiring Diagrams 11. Torque Specifications

General Specifications

Key technical data for 1987-1990 Yamaha Jog models.

Specification	Details
Engine Type	2-stroke, air-cooled, single cylinder
Displacement	49cc
Bore x Stroke	40.0mm x 39.2mm
Compression Ratio	7.0:1
Carburetor	Mikuni VM12SH
Ignition System	CDI
Transmission	Automatic V-belt
Fuel Capacity	4.0 liters
Oil Capacity	1.2 liters
Dry Weight	58 kg

Maintenance Schedule

Recommended service intervals for optimal performance.

Engine Oil: Change every 1,000 km or 6 months
Spark Plug: Replace every 3,000 km
Air Filter: Clean every 1,500 km, replace every 6,000 km
Drive Belt: Inspect every 3,000 km, replace at signs of wear
Brake Pads: Check every 2,000 km
Wheel Bearings: Lubricate every 5,000 km

WARNING! Always use genuine Yamaha parts for replacement. Follow safety procedures when working with fuel and electrical systems.

Engine Service

Complete engine disassembly and reassembly procedures.

Cylinder Head Removal: Remove 4 bolts in crisscross pattern
Cylinder/Piston: Inspect for scoring, measure ring end gap
Crankcase Splitting: Mark halves before separation
Crankshaft: Check runout and bearing play
Reed Valve: Inspect for cracks or warping

CAUTION! Use proper torque specifications during reassembly. Clean all parts thoroughly before installation.

Fuel System

Carburetor: Float level 4.5mm, main jet #75, pilot jet #35
Fuel Pump: Check for proper pressure and flow
Oil Injection: Verify pump synchronization with throttle
Fuel Lines: Inspect for cracks and replace if necessary
Air Filter: Clean with compressed air, oil lightly

Electrical System

Complete electrical troubleshooting and component testing.

CDI Unit: Test resistance values per specifications. Stator: Check coil resistance: source coil 0.2-1.0Ω, pickup coil 20-250Ω. Voltage Regulator: Output 13.5-15V DC. Battery: 12V 3Ah, maintain proper electrolyte level.

Transmission

Automatic V-belt drive system service procedures.

Drive Belt: Remove variator cover, check for cracks and wear
Variator: Inspect rollers for flat spots, replace if worn
Clutch: Check shoes for wear, clean dust from bell housing
Drive Face: Lubricate with high-temperature grease

Tip: Always replace drive belt and rollers as a set for optimal performance.

Brake System

Front and rear brake maintenance and repair.

Brake Pads: Measure thickness, replace if below 1.5mm
Brake Shoes: Check for contamination and wear
Cables: Lubricate and adjust free play
Drum Brakes: Clean and inspect for scoring
Brake Lever: Adjust to proper height and tension

WARNING! Test brakes thoroughly after service. Never ride with worn brake components.

Suspension

Front fork and rear shock absorber service.

Front Forks: Check oil level (85-95mm from top), replace seals if leaking. Rear Shock: Inspect for oil leaks, check spring preload adjustment. Wheel Alignment: Ensure proper alignment to prevent uneven tire wear.

Troubleshooting

Symptom	Possible Cause	Corrective Action
Hard starting	Fouled spark plug, weak compression	Replace spark plug, check compression (should be 120+ psi)
Poor acceleration	Clogged carburetor, worn drive belt	Clean carburetor jets, replace drive belt
No spark	Faulty CDI, bad stator	Test ignition components, replace as needed
Overheating	Lean fuel mixture, clogged cooling fins	Adjust carburetor, clean engine fins
Transmission slip	Worn variator rollers, glazed clutch	Replace rollers, sand clutch shoes lightly

Wiring Diagrams

Complete electrical schematics for all model years. Color-coded wiring with connector locations. Main harness routing and ground points identified. CDI wiring specifications and testing procedures included.

Torque Specifications

Critical fasteners and proper torque values.

Component	Torque Value
Cylinder Head Bolts	18 Nm
Spark Plug	25 Nm
Drive Face Nut	55 Nm
Axle Nuts	60 Nm
Engine Mount Bolts	40 Nm

Service Note: Use calibrated torque wrench for all critical fasteners.

Parts Support: Contact authorized Yamaha dealer for genuine parts.