1987 YZ80 Service Manual

The 1987 YZ80 is a high-performance 80cc motocross motorcycle featuring a liquid-cooled two-stroke engine, 6-speed transmission, front and rear suspension systems, disc brakes, and competition-oriented chassis design. This service manual provides comprehensive technical information for maintenance, repair, and troubleshooting procedures. Below are key sections covering specifications, maintenance schedules, engine service, chassis components, and troubleshooting guidelines.

General Specifications

Technical specifications and performance data for the 1987 YZ80 motocross motorcycle.

Specification	Description
Engine Type	Liquid-cooled 2-stroke single cylinder
Displacement	79cc
Bore x Stroke	47.0 x 45.6mm
Compression Ratio	8.3:1
Carburetor	Mikuni VM24SS
Ignition System	CDI
Transmission	6-speed constant mesh
Final Drive	Chain
Suspension Front	Telescopic fork
Suspension Rear	Monoshock swingarm
Brakes Front	Hydraulic disc
Brakes Rear	Hydraulic disc
Dry Weight	68 kg (150 lbs)
Fuel Capacity	5.5 liters (1.45 gallons)

Maintenance Schedule

Recommended maintenance intervals for optimal performance and longevity.

1. Before each ride: Check fuel level, tire pressure, chain tension, brake operation.
2. Every 10 hours: Clean air filter, check spark plug, inspect brakes and cables.
3. Every 20 hours: Change transmission oil, check wheel bearings, inspect suspension.
4. Every 50 hours: Replace spark plug, clean carburetor, check valve clearances.
5. Every 100 hours: Top end rebuild, replace piston and rings, inspect crankshaft.

WARNING! Always use proper safety equipment and follow torque specifications. Engine must be cool before performing maintenance.

Engine Service

Detailed procedures for engine disassembly, inspection, and reassembly.

1. Cylinder Head Removal: Remove cooling system, exhaust pipe, and cylinder head bolts.
2. Cylinder Removal: Remove cylinder base nuts and carefully lift cylinder off crankcase.
3. Piston Service: Remove circlips, wrist pin, and piston. Inspect for wear and damage.
4. Crankcase Splitting: Remove all external components and split cases using proper tools.
5. Crankshaft Service: Inspect bearings and seals. Replace if worn or damaged.
6. Reassembly: Use new gaskets and seals. Follow torque sequence and specifications.

CAUTION! Clean work area thoroughly. Use factory service tools for critical operations.

Fuel System

Carburetor service and fuel system maintenance procedures.

• Carburetor Cleaning: Disassemble and clean all jets and passages.
• Float Level: Adjust to 18.5mm ± 1.0mm from float bowl mating surface.
• Jet Specifications: Main jet #150, Pilot jet #35, Needle clip 2nd position.
• Fuel Mix Ratio: 32:1 (4 ounces of 2-stroke oil per gallon of gasoline).
• Fuel Filter: Replace if contaminated or restricted.
• Throttle Cable: Adjust for 2-3mm free play at throttle grip.

Electrical System

Ignition, charging, and lighting system service procedures.

CDI System: Check resistance values: Primary coil 0.1-0.3Ω, Secondary coil 3.5-4.7kΩ.

Spark Plug: NGK BR9ES, gap 0.6-0.7mm.

Stator: Lighting coil resistance 0.1-1.0Ω, Source coil resistance 240-360Ω.

Voltage Regulator: Test continuity between terminals.

Wiring Harness: Inspect for chafing, corrosion, or damaged connectors.

Chassis Components

Frame, swingarm, and steering head service procedures.

1. Frame Inspection: Check for cracks, bends, or damage at stress points.
2. Swingarm Service: Remove and inspect bearings. Replace if worn.
3. Steering Head: Adjust bearing preload for smooth operation without play.
4. Footpegs: Check mounting bolts and return springs.
5. Exhaust System: Inspect for leaks and secure mounting.

Tip: Use thread locking compound on all critical fasteners.

Suspension System

Front fork and rear shock service procedures.

1. Front Fork Disassembly: Drain oil, remove springs, inspect bushings and seals.
2. Fork Oil Capacity: 260ml per leg (10W weight recommended).
3. Fork Seal Replacement: Use proper seal driver tool for installation.
4. Rear Shock Service: Check nitrogen pressure, inspect shaft for wear.
5. Suspension Settings: Adjust preload and damping for rider weight and conditions.
6. Linkage Bearings: Grease and inspect for wear during service.

WARNING! Use proper tools for spring compression. Wear eye protection.

Brake System

Front and rear hydraulic disc brake service.

• Brake Pads: Replace when worn to wear indicators.
• Brake Fluid: DOT 4, change annually or if contaminated.
• Bleeding Procedure: Use clear tubing and fresh fluid.
• Caliper Service: Clean and lubricate sliding pins.
• Rotor Inspection: Check for warpage and minimum thickness.
• Master Cylinder: Inspect reservoir and piston seals.

Wheels and Tires

Wheel service and tire installation procedures.

Wheel Bearing Replacement: Remove seals, press out old bearings, install new bearings.

Tire Installation: Use tire irons and rim protectors. Match rotation direction arrows.

Tire Pressures: Front 12-15 psi, Rear 10-12 psi (adjust for conditions).

Spoke Tension: Check and adjust evenly around wheel circumference.

Rim Inspection: Check for dents, cracks, or runout.

Troubleshooting

Symptom	Possible Cause	Corrective Action
Hard starting	Fouled plug, stale fuel	Replace spark plug; drain old fuel; check compression.
Poor performance	Clogged carburetor, worn piston	Clean carburetor jets; inspect top end; check reed valves.
Transmission issues	Worn gears, bent shift fork	Inspect transmission components; replace damaged parts.
Brake fade	Contaminated fluid, worn pads	Bleed brakes; replace pads; clean rotors.
Suspension bottoming	Low oil level, worn springs	Change fork oil; adjust preload; replace springs if necessary.
Chain problems	Improper tension, worn sprockets	Adjust chain slack; replace chain and sprockets as set.

Technical Support: Consult authorized Yamaha dealer for complex issues.

Parts Information: Use genuine Yamaha parts for optimal performance and reliability.