🏷️ Basic Information

Make:

Yamaha

Model:

YZ125

Year:

2004

Vehicle Type:

Motocross Motorcycle

Engine Code:

5NY

Chassis Code:

5JY

👋 Introduction

Segment:

Competitive Open Class (125cc two-stroke) motocross segment.

Target Audience:

Motocross racers, off-road enthusiasts, and mechanics specializing in Yamaha YZ models.

Key Competitors 2004:

KTM 125 SX, Honda CR125R, Suzuki RM125, Kawasaki KX125.

Engine Type:

Liquid-cooled, 124cc, 2-stroke, reed valve, YPVS (Yamaha Power Valve System) equipped.

Suspension Front:

Upside-down telescopic fork (KYB AOS, 46mm diameter), adjustable compression and rebound damping.

Suspension Rear:

Monocross suspension with fully adjustable KYB shock absorber.

Brakes:

Hydraulic disc brakes front and rear.

Completeness:

Provides a complete guide for all maintenance, repair, and troubleshooting tasks.

Illustration Quality:

Features clear, detailed, and accurate illustrations for every procedure.

📋 Manual Details

Systems Covered:

Engine, Transmission, Chassis, Suspension, Brakes, Electrical System, Fuel System, Cooling System, Exhaust System, Lubrication System, Bodywork, Troubleshooting, Specifications, Maintenance

Page Count:

Estimated 300-400 pages (typical for this era of service manual)

Manual Type:

Service Repair Workshop Manual

Language:

English

Format:

Downloadable PDF

📖 Manual Coverage

Engine Service:

Detailed overhaul procedures for top end (cylinder, piston, rings, head), bottom end (crankshaft, connecting rod, bearings), and all associated components. Includes timing specifications and assembly sequences.

Transmission Service:

Complete instructions for gearbox disassembly, inspection, repair, and reassembly. Covers clutch operation, shift mechanism, and gear selection troubleshooting.

Suspension Service:

Front fork (KYB AOS type) and rear shock absorber (Monocross system) disassembly, service, and adjustment procedures. Includes oil types, capacities, and damping adjustments.

Brake Service:

Front and rear hydraulic disc brake system maintenance, including caliper overhaul, pad replacement, fluid bleeding, master cylinder service, and rotor inspection.

Electrical System:

Wiring diagrams, component testing (ignition, lighting, sensors), troubleshooting guides for electrical faults, and CDI unit information.

Fuel System:

Carburetor (Mikuni VM26SS) disassembly, cleaning, adjustment, and jetting information. Fuel tank, petcock, and fuel line maintenance.

Cooling System:

Radiator maintenance, coolant replacement, and troubleshooting of the liquid cooling system.

Lubrication System:

Two-stroke pre-mix oil ratios, gearbox oil specifications, and lubrication points for moving parts.

Bodywork And Frame:

Fairing removal and installation, seat, fuel tank, and chassis component inspection and maintenance.

Scheduled Maintenance:

Detailed procedures for routine checks, cleaning, lubrication, and adjustments at specified intervals.

Troubleshooting Guides:

Systematic diagnostic steps for identifying and resolving common mechanical and electrical problems.

Engine Rebuild:

Comprehensive, step-by-step instructions for complete engine disassembly, inspection of wear limits, and rebuilding.

Carburetor Tuning:

Guidance on adjusting the carburetor for different altitudes and conditions, including jet changes.

Suspension Setup:

Information on adjusting suspension sag, compression, and rebound damping for rider preference and terrain.

Wheel And Tire Service:

Instructions for wheel bearing maintenance, spoke tensioning, and tire changes.

Electrical Diagnostics:

Procedures for testing ignition coil, spark plug, CDI unit, and wiring harness continuity.

Clutch Adjustment And Replacement:

Detailed steps for adjusting clutch lever free play and replacing clutch plates and springs.

Depth Of Coverage:

Expert-level, covering everything from basic maintenance to full engine and transmission rebuilds. Includes detailed torque specifications and tolerances.

Diagnostic Capability:

Provides comprehensive troubleshooting flowcharts and diagnostic procedures for all major systems.

Diagrams And Illustrations:

Richly illustrated with clear, exploded-view diagrams and step-by-step photos for component identification and assembly.

Specification Accuracy:

Features precise torque values, clearance measurements, fluid capacities, and electrical specifications crucial for proper repair.

📊 Manual Specifications

File Format:

PDF

Searchable Text:

Likely includes OCR (Optical Character Recognition) for text searchability.

Print Quality:

High-resolution, suitable for printing individual pages or the entire manual.

Binding Type:

N/A (Digital Download)

Paper Type:

N/A (Digital Download)

⚙️ Engine Specifications

Engine Type:

2-stroke, Liquid-cooled, Reed valve, Single cylinder

Displacement:

124 cc

Bore X Stroke:

54.0 mm x 54.5 mm

Compression Ratio:

8.6:1 (at 1400 rpm)

Carburetor:

Mikuni VM26SS

Ignition:

CDI (Capacitor Discharge Ignition)

Ypvs System:

Yamaha Power Valve System, electrically actuated for optimized power delivery across RPM range.

Horsepower Est:

Approx. 35-40 HP (at crankshaft, depending on tuning and exhaust)

Torque Est:

Not officially published, but significant for its class.

Fuel Type:

Unleaded gasoline (min. 90 octane recommended), mixed with 2-stroke oil.

Fuel Mixture Ratio:

50:1 (Yamalube 2R or equivalent 2-stroke oil)

Fuel Tank Capacity:

8.0 Liters (2.1 Gallons)

🔧 Transmission

Transmission Type:

6-speed constant mesh

Clutch Type:

Wet, multiple-disc

Final Drive:

Chain drive

Gear Ratios:

Detailed in manual (typically: 1st: 2.xxx, 2nd: 1.xxx, 3rd: 1.xxx, 4th: 1.xxx, 5th: 1.xxx, 6th: 1.xxx)

Transmission Oil Type:

Yamalube 10W-30 API SG or SAE 80 API GL-4 Gear Oil

Transmission Oil Capacity:

0.70 Liters (0.74 US qt)

📐 Dimensions

Overall Length:

2120 mm (83.5 in)

Overall Width:

820 mm (32.3 in)

Overall Height:

1280 mm (50.4 in)

Seat Height:

970 mm (38.2 in)

Wheelbase:

1475 mm (58.1 in)

Dry Weight:

93 kg (205 lbs)

Wet Weight:

Approx. 99 kg (218 lbs) (with all fluids)

Front Tire:

80/100-21M/C 51M (Tube type)

Rear Tire:

110/90-19M/C 62M (Tube type)

Front Wheel:

21 x 1.60

Rear Wheel:

19 x 1.85

Front Suspension Travel:

300 mm (11.8 in)

Rear Suspension Travel:

320 mm (12.6 in)

🛡️ Safety Features

Safety Standards:

N/A (Motocross competition vehicle, not street-legal)

Braking System:

Powerful hydraulic disc brakes offer strong stopping power, essential for off-road control.

Frame Construction:

Lightweight and rigid aluminum frame provides precise handling and rider feedback.

💡 Technology Features

Infotainment Features:

N/A (Performance motocross vehicle)

Assistance Features:

N/A (Performance motocross vehicle)

Connectivity Features:

N/A (Performance motocross vehicle)

🔧 Maintenance

Air Filter Cleaning:

After every ride (recommended)

Spark Plug Inspection:

Every 10-15 hours of use

Chain Lubrication:

Every ride, or as needed

Clutch Adjustment:

Periodically, as free play changes

Engine Top End Rebuild:

Varies greatly based on riding conditions and intensity, typically 30-50 hours for competitive use.

Coolant Type:

Yamaha Coolant (or equivalent ethylene glycol-based coolant with corrosion inhibitors)

Fork Oil Type:

Yamaha Fork Oil G10 (or SAE 0W-20 equivalent)

Fork Oil Level:

130 mm (5.12 in) from top of fork tube when compressed

Shock ABSorber Oil:

Specific Yamaha shock oil (refer to manual for details)

Common Wear Items:

Piston rings, piston, clutch plates, wheel bearings, chain and sprockets, brake pads.

Ypvs Cleaning:

The YPVS power valve system can accumulate carbon deposits, requiring periodic cleaning to maintain optimal performance.

Carburetor Jetting:

Proper carburetor jetting is critical for performance and longevity, especially when riding at different altitudes or temperatures.

📚 Vehicle History

Yamaha Motorsport Legacy:

Yamaha has a long and successful history in motocross, with the YZ line being a dominant force for decades.

Yz125 Development:

The YZ125 has undergone continuous evolution, with significant updates in chassis, suspension, and engine technology throughout the 1990s and early 2000s.

2004 Model Updates:

The 2004 model typically saw refinements in suspension settings, engine tuning, and minor chassis updates to maintain competitiveness.

Manufacturing Location:

Japan

🏆 Awards & Recognition

Motocross Achievements:

The YZ125 has a long history of success in amateur and professional motocross racing, with numerous championships won by riders using this model.