1988 YZ 125 Manual

The 1988 Yamaha YZ 125 is a high-performance motocross motorcycle featuring a liquid-cooled 125cc two-stroke engine, 6-speed transmission, front and rear disc brakes, adjustable suspension, and lightweight aluminum frame. This manual covers maintenance, operation, troubleshooting, and specifications for optimal performance and longevity.

Features at a Glance

Key components: Liquid-cooled 125cc two-stroke engine, 6-speed transmission, front and rear disc brakes, adjustable suspension, aluminum frame.

Feature	Description
Engine Type	Liquid-cooled 2-stroke single cylinder
Displacement	125cc
Transmission	6-speed constant mesh
Front Brake	Single disc brake
Rear Brake	Single disc brake
Front Suspension	Telescopic fork, adjustable
Rear Suspension	Monoshock, adjustable
Frame	Aluminum semi-double cradle
Fuel Capacity	2.1 gallons (8 liters)
Weight	194 lbs (88 kg) dry

Installation & Assembly

Unpack and inspect all components. Assemble on level surface with proper tools.

1. Handlebar installation: Mount handlebars to triple clamp, torque to specification.
2. Wheel installation: Install front and rear wheels, align axles properly.
3. Control connections: Connect throttle, clutch, and brake cables.
4. Fluid filling: Add recommended engine oil and coolant.

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

First-Time Setup

Complete pre-ride inspection and adjustments before operation.

1. Fuel mixture: Prepare 32:1 fuel/oil mixture with premium gasoline.
2. Suspension setup: Adjust preload and damping for rider weight.
3. Control adjustment: Set lever positions and free play.
4. Tire pressure: Inflate to recommended pressures (12-14 PSI).

CAUTION! Always wear proper protective gear. Break in engine gradually.

Specifications

• Engine: 125cc liquid-cooled 2-stroke
• Bore x Stroke: 56.0 x 50.7 mm
• Compression Ratio: 8.5:1
• Carburetor: Mikuni VM34SS
• Ignition: CDI
• Transmission: 6-speed
• Final Drive: Chain 520-112 links
• Suspension Travel: Front 11.8 in, Rear 12.2 in
• Ground Clearance: 13.0 in
• Seat Height: 37.4 in

Controls and Operation

Familiarize yourself with all controls before riding.

Left Side: Clutch lever, horn button, turn signal switch, high/low beam.

Right Side: Front brake lever, throttle, engine stop switch, start button.

Operation: Start with choke, warm up engine, use clutch smoothly, shift through gears progressively. Always use both brakes for stopping.

Maintenance Procedures

Regular maintenance ensures optimal performance and safety.

1. Pre-ride inspection: Check tires, controls, fluids, fasteners.
2. Air filter: Clean every 10 hours or as needed.
3. Chain adjustment: Maintain 1.2-1.6 inches free play.
4. Brake pads: Inspect wear regularly, replace when necessary.

Tip: Keep maintenance log for service intervals.

Engine Service

Proper engine maintenance critical for performance.

1. Spark plug: NGK BR9ES, gap 0.024-0.028 in.
2. Coolant: Change annually, use ethylene glycol mixture.
3. Top end: Inspect piston and rings every 40 hours.
4. Carburetor: Clean jets regularly, adjust air screw as needed.
5. Power valve: Clean and inspect every 20 hours.

WARNING! Engine components get extremely hot. Allow to cool before servicing.

Suspension & Brakes

Adjust suspension for riding conditions and skill level.

Front fork: Adjust compression and rebound damping. Rear shock: Adjust preload, compression, rebound. Brakes: Use DOT 4 fluid, bleed system annually. Check pad wear regularly.

Care & Cleaning

Clean after every ride to prevent corrosion and damage.

Use mild soap and water, avoid high-pressure spray near bearings and seals. Lubricate chain after cleaning. Store in dry, covered area.

CAUTION! Do not use harsh chemicals that may damage plastics or paint.

Regulatory Information

Designed for closed-course competition use only. Not legal for street use. Meets applicable noise and emission standards for competition vehicles. Copyright © 1988 Yamaha Motor Corporation. For support, contact authorized Yamaha dealer.

Troubleshooting

Symptom	Possible Cause	Corrective Action
Hard starting	Fouled plug, stale fuel	Replace spark plug, drain old fuel mixture.
Poor performance	Clogged air filter, carb issues	Clean air filter, inspect carburetor jets.
Overheating	Low coolant, lean mixture	Check coolant level, adjust fuel mixture.
Brake fade	Contaminated pads, old fluid	Replace pads, bleed brake system.
Transmission issues	Low oil, worn clutch	Check oil level, inspect clutch plates.

Service: Contact authorized Yamaha dealer for major repairs.