1987 YZ 125 Manual

The 1987 Yamaha YZ 125 is a high-performance motocross motorcycle featuring a liquid-cooled 125cc two-stroke engine, 6-speed transmission, monoshock rear suspension, and advanced chassis design for competitive racing. This manual covers maintenance, specifications, operation, and troubleshooting procedures to keep your YZ 125 in peak racing condition.

1. Features at a Glance 2. Installation & Assembly 3. First-Time Setup 4. Specifications 5. Controls and Settings 6. Maintenance Procedures 7. Engine Operation 8. Safety Features 9. Care & Cleaning 10. Regulatory Information 11. Troubleshooting

Features at a Glance

Key components: Liquid-cooled 125cc two-stroke engine, 6-speed transmission, monoshock rear suspension, front disc brake, rear drum brake, aluminum swingarm.

Feature	Description
Engine Type	Liquid-cooled 2-stroke single cylinder
Displacement	125cc
Transmission	6-speed constant mesh
Front Suspension	Telescopic fork
Rear Suspension	Monoshock with adjustable preload
Front Brake	Hydraulic disc
Rear Brake	Mechanical drum
Fuel Capacity	2.1 gallons (8 liters)
Dry Weight	194 lbs (88 kg)
Seat Height	36.6 inches (93 cm)

Installation & Assembly

Unpack and inspect contents. Assemble on stable surface.

Install handlebars: Position and tighten clamp bolts to specification.
Mount front wheel: Insert axle and tighten to proper torque.
Install fuel tank: Secure with mounting bolts and connect fuel line.
Attach controls: Install throttle, clutch, and brake levers.
Add fluids: Fill with recommended 2-stroke oil and fuel mixture.

WARNING! Motorcycle is heavy; use proper lifting techniques. Ensure all fasteners are torqued to specification.

First-Time Setup

Complete assembly and perform initial setup procedures.

Check tire pressure: Inflate to recommended PSI (front: 12-15, rear: 10-12).
Adjust suspension: Set sag and damping for rider weight and conditions.
Lubricate chain: Apply quality chain lubricant.
Check fluid levels: Verify engine oil, coolant, and brake fluid.
Test controls: Ensure smooth operation of throttle, clutch, and brakes.

CAUTION! Always wear proper safety gear during setup and operation.

Specifications

Engine: 125cc liquid-cooled 2-stroke
Bore x Stroke: 56.0 x 50.7 mm
Compression Ratio: 8.2:1
Carburetor: Mikuni VM34SS
Ignition: CDI
Transmission: 6-speed wet clutch
Final Drive: Chain 520-112 links
Front Tire: 80/100-21
Rear Tire: 100/90-19
Ground Clearance: 13.4 inches (34 cm)

Controls and Settings

Familiarize yourself with all controls before operation.

Right Handlebar: Front brake lever, throttle grip, engine kill switch

Left Handlebar: Clutch lever, horn button, light switch

Instrumentation: Speedometer, tachometer, neutral indicator, high beam indicator

Adjustable Settings: Suspension preload, brake lever position, clutch free play, idle speed

Maintenance Procedures

Regular maintenance ensures optimal performance and longevity.

Pre-ride inspection: Check tires, controls, fluids, and fasteners.
Engine oil change: Every 15 hours or as needed.
Air filter cleaning: After every ride in dusty conditions.
Chain adjustment: Maintain 1.2-1.6 inches (30-40mm) free play.
Spark plug replacement: Every 25 hours or as needed.
Coolant change: Annually or every 50 hours.

Tip: Keep detailed maintenance records for reference.

Engine Operation

Proper operation techniques for 2-stroke performance.

Starting procedure: Pull choke, kick start with smooth motion.
Warm-up: Allow engine to reach operating temperature before riding.
Shifting: Use clutch smoothly, match engine RPM to road speed.
Power band: Familiarize yourself with peak power range (8000-11000 RPM).
Fuel mixture: Use premium gasoline with 32:1 oil ratio.
Cool down: Allow engine to idle before shutdown after hard riding.

WARNING! Always operate in well-ventilated areas to avoid carbon monoxide poisoning.

Safety Features

Engine kill switch: Immediate engine shutdown. Brake lights: Alert following vehicles. Reflectors: Improve visibility. Proper gear: Always wear helmet, gloves, boots, and protective clothing.

Care & Cleaning

Clean after every ride to prevent corrosion and maintain appearance.

Use mild soap and water for general cleaning. Avoid high-pressure spray on bearings and seals. Lubricate chain after cleaning. Wax painted surfaces for protection.

CAUTION! Do not use harsh chemicals that may damage finishes or components.

Regulatory Information

EPA compliant for off-road use only. Not legal for street operation. Meets all applicable noise and emission standards for 1987 model year. Copyright © 1987 Yamaha Motor Corporation; all rights reserved. For technical support, contact Yamaha customer service or authorized dealers.

Troubleshooting

Symptom	Possible Cause	Corrective Action
Hard starting	Fouled plug, stale fuel	Replace spark plug; drain old fuel; check compression.
Poor performance	Clogged air filter, jetting	Clean air filter; check carburetor jets; verify ignition timing.
Overheating	Low coolant, lean mixture	Check coolant level; verify fuel mixture; clean radiator.
Transmission issues	Low oil, worn clutch	Check oil level; inspect clutch plates; adjust cable.
Brake problems	Air in lines, worn pads	Bleed brakes; replace pads; check rotor condition.

Reset: For electrical issues, disconnect battery for 30 seconds.

Service: Contact authorized Yamaha dealer for complex repairs.