🏷️ Basic Information

Make:

Yamaha

Model:

YZ125

Year:

1987

Model Code:

YZ125U

👋 Introduction

Market Segment:

Motocross / Off-Road Competition

Competitors 1987:

Honda CR125R, Suzuki RM125, Kawasaki KX125

Performance Focus:

Lightweight, agile, and powerful for its displacement class, designed for competitive motocross racing.

Value Proposition:

Essential resource for owners and technicians to maintain, diagnose, and repair the 1987 Yamaha YZ125 to factory standards.

📋 Manual Details

Systems Covered:

Engine, Fuel System, Ignition System, Cooling System, Transmission, Clutch, Suspension (Front & Rear), Brakes (Front & Rear), Wheels & Tires, Electrical System, Chassis, Frame, Bodywork, Lubrication, Troubleshooting, Maintenance

Page Count:

Approximately 300-400 pages (estimated based on typical Yamaha service manuals of this era)

Manual Type:

Service Repair Workshop Manual

Language:

English

Format:

Digital (PDF - assumed, common for sales)

📖 Manual Coverage

Engine Service:

Complete engine disassembly, assembly, inspection, and repair procedures for the 123cc 2-stroke engine.

Fuel System Service:

Carburetor (Mikuni VM series) tuning, cleaning, rebuilding, and specifications; fuel tank and petcock maintenance.

Ignition System Service:

CDI ignition system diagnosis, testing, and component replacement (stator, coil, spark plug).

Cooling System Service:

Air cooling system inspection and maintenance. (Note: YZ125 models of this era were air-cooled).

Transmission Clutch Service:

Gearbox disassembly, inspection, and repair; clutch assembly inspection, adjustment, and replacement (plates, springs).

Suspension Service:

Front fork (Telescopic) oil change, seal replacement, damping adjustment; rear monoshock (Monocross) linkage service, shock absorber overhaul, and spring preload adjustment.

Brake Service:

Front and rear disc brake system maintenance, caliper overhaul, master cylinder service, pad replacement, and bleeding procedures.

Electrical System Service:

Wiring diagrams, lighting system, battery maintenance, fuse replacement, and component testing.

Chassis Frame Bodywork Service:

Frame inspection, steering head bearing service, swingarm lubrication, chain and sprocket maintenance, body panel removal/installation.

Routine Maintenance:

Detailed instructions for regular checks, adjustments, and replacements (e.g., air filter cleaning, spark plug replacement, chain adjustment).

Troubleshooting Guides:

Systematic diagnostic procedures for identifying and resolving common and uncommon operational issues.

Rebuild Procedures:

Step-by-step instructions for complete engine, carburetor, and suspension component overhauls.

Technical Specifications:

Torque values, clearances, fluid capacities, electrical specifications, and wear limits for all major components.

Wiring Diagrams:

Comprehensive electrical system schematics for accurate diagnosis and repair.

Depth Of Coverage:

Comprehensive, covering all aspects from basic user maintenance to advanced engine rebuilding and troubleshooting.

📊 Manual Specifications

File Format:

PDF (Portable Document Format)

Searchability:

Likely text-searchable if digitally generated from original documents.

Print Quality:

High-resolution scans or original digital files ensuring clear diagrams and text.

Binding Type:

N/A (Digital format)

Paper Type:

N/A (Digital format)

Completeness:

Full workshop manual, encompassing all aspects of repair and maintenance.

⚙️ Engine Specifications

Engine Type:

2-Stroke, Single Cylinder

Displacement:

123 cc (cc)

Bore X Stroke:

56.0 mm x 50.0 mm (typical for this model)

Cooling System:

Air-Cooled

Compression Ratio:

Approximately 7.9:1 (typical, refer to manual for exact spec)

Horsepower:

Approximately 28-30 HP (estimated at crankshaft, varies by region and tuning)

Torque:

Not officially published in user manuals, but significant for its class.

Carburetor Type:

Mikuni VM28SS (typical)

Fuel Type:

Unleaded Gasoline (Octane rating as specified in manual)

Fuel Mixture:

Requires 2-stroke oil pre-mix (ratio specified in manual, typically 20:1 or 32:1)

Ignition Type:

CDI (Capacitor Discharge Ignition)

Spark Plug Type:

NGK BR9ES (typical, verify in manual)

🔧 Transmission

Transmission Type:

6-Speed Constant Mesh

Clutch Type:

Wet, Multi-plate

Final Drive:

Chain Drive

Transmission Oil Type:

Yamaha Gear Oil or equivalent (refer to manual for viscosity and specification, e.g., SAE 80 or SAE 10W-30)

Transmission Oil Capacity:

Approximately 0.7 Liters (refer to manual for exact capacity)

📐 Dimensions

Length:

Approx. 2120 mm (7' 3.5") (verify in manual)

Width:

Approx. 820 mm (2' 8.3") (verify in manual)

Height:

Approx. 1260 mm (4' 1.6") (verify in manual)

Wheelbase:

Approx. 1450 mm (57.1") (verify in manual)

Dry Weight:

Approximately 90 kg (198 lbs) (verify in manual)

Fuel Tank Capacity:

Approximately 7.0 Liters (1.8 US gal) (verify in manual)

Front Suspension Travel:

Approx. 280 mm (11.0") (verify in manual)

Rear Suspension Travel:

Approx. 300 mm (11.8") (verify in manual)

Front Tire Size:

80/100-21 46M (verify in manual)

Rear Tire Size:

100/90-18 56M (verify in manual)

Front Brake Type:

Disc

Rear Brake Type:

Disc

🛡️ Safety Features

Safety Standards:

Designed for closed-course competition; safety features focus on rider control and protection during aggressive riding.

Braking System:

Hydraulic disc brakes front and rear for effective stopping power.

Suspension Performance:

Monocross rear suspension and telescopic front forks provide stability and control over varied terrain.

💡 Technology Features

Infotainment System:

N/A (Competition motocross bike)

Driver Assistance Features:

N/A (Focus on rider control, not electronic assistance)

🔧 Maintenance

Oil Change Interval:

Refer to manual; typically after specific operating hours or event types (e.g., racing).

Air Filter Cleaning:

Frequent cleaning and re-oiling recommended, especially in dusty conditions.

Spark Plug Replacement:

As per manual, typically based on hours of use or performance degradation.

Chain Lubrication Adjustment:

Regularly (before/after each ride) depending on conditions.

Brake Fluid:

DOT 3 or DOT 4 Brake Fluid (refer to manual for specific type and grade)

Fork Oil Type:

Yamalube Fork Oil G10 or equivalent (refer to manual for viscosity)

Coolant:

N/A (Air-cooled)

Carburetor Clogging:

Common in 2-strokes due to fuel quality or sediment. Regular cleaning is key.

Clutch Wear:

Can wear faster with aggressive riding; manual details inspection and replacement.

Suspension Seals:

Fork seal leaks are common over time; manual provides replacement instructions.

Engine Top End:

Piston and rings require periodic replacement based on usage and maintenance.

📚 Vehicle History

Yamaha Motorsports:

Yamaha has a long-standing legacy in motorsports, particularly in motocross, with a history of innovation and performance.

Yz Series Development:

The YZ125 has been a cornerstone of Yamaha's motocross lineup, undergoing continuous development to improve power, handling, and suspension technology.

Manufacturing Location:

Japan (Typical for Yamaha)

Production Years:

Ongoing since the 1970s, with significant updates throughout the decades.

🏆 Awards & Recognition

Motocross Awards:

The YZ125, in its various iterations, has consistently been a competitive and well-regarded machine in professional motocross and supercross championships.