🏷️ Basic Information

Make:

Yamaha

Model:

YZ125

Model Variant:

YZ125

Year Covered:

2004

Chassis Code:

4JT

Product Id:

YZ125-2004-SRM

Oem Manual Code:

Likely 4JT-28197-E0 or similar (specific OEM code not provided, but representative of Yamaha documentation)

👋 Introduction

Segment:

Motocross Competition (Open Class 125cc)

Target Audience:

Motocross racers, performance enthusiasts, and dedicated Yamaha YZ125 owners.

Key Competitors:

Honda CR125R, Kawasaki KX125, Suzuki RM125.

Vehicle Focus:

Lightweight, agile, powerful 125cc two-stroke engine, advanced suspension for motocross riding.

Manual Value Proposition:

Empowers owners with the knowledge to maintain peak performance and conduct complex repairs, maximizing the lifespan and capabilities of the YZ125.

📋 Manual Details

Coverage Scope:

Comprehensive service and repair for the 2004 Yamaha YZ125.

Applicable Years:

Primarily 2004, with noted compatibility for similar years and models (e.g., 1985 mentioned, though significant differences exist; this manual is specific to the 2004 generation).

Estimated Page Count:

300-400 pages (typical for this era's comprehensive service manuals)

Type:

Service Repair Workshop Manual

Language:

English

Format:

Digital (likely PDF), searchable

📖 Manual Coverage

Engine:

Top-end rebuild, bottom-end assembly, cooling system (air-cooled), lubrication, ignition system, carburetor tuning and overhaul.

Transmission:

Clutch service, gearshift mechanism, gearbox disassembly and reassembly, lubrication.

Suspension:

Front fork (Kayaba) disassembly, seal replacement, oil levels and specifications, rear shock absorber (Monoshock) service and linkage maintenance.

Brakes:

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

Electrical:

Ignition system wiring diagram, lighting, CDI unit, high-tension lead, stator and rotor inspection.

Fuel System:

Fuel tank, petcock, fuel lines, carburetor (Mikuni TM38) jetting and component service.

Cooling System:

Air-cooled engine design principles, cylinder fin maintenance for optimal heat dissipation.

Chassis And Frame:

Frame inspection, steering head bearing service, sub-frame maintenance, footpeg and control lever adjustments.

Exhaust System:

Pipe and silencer inspection, mounting, and servicing.

Routine Maintenance:

Oil changes, air filter cleaning/replacement, spark plug inspection/replacement, chain adjustment, bolt torque checks.

Troubleshooting:

Diagnosis of common performance issues, starting problems, engine noise, and electrical faults with step-by-step guides.

Engine Rebuild:

Complete engine disassembly, cylinder honing, piston and ring replacement, crankshaft inspection and reassembly, bearing replacement.

Suspension Overhaul:

Detailed instructions for fork seal replacement, damper rod service, and rear shock absorber rebuild.

Carburetor Tuning:

Jetting recommendations, float bowl adjustment, idle speed setting, mixture screw settings for various conditions.

Electrical Diagnostics:

Testing of CDI unit, stator output, and wiring continuity using a multimeter.

Brake System Service:

Pad replacement, caliper piston retraction, master cylinder seal replacement, complete brake fluid flush and bleeding.

Depth Level:

Expert-level, suitable for professional mechanics and experienced DIY enthusiasts.

Detail Level:

Includes detailed torque specifications, clearance measurements, assembly sequences, and special tool requirements.

📊 Manual Specifications

File Type:

PDF

Searchability:

Text-searchable for quick information retrieval.

Printability:

Printable pages for workshop use.

Image Resolution:

High resolution for clear diagrams and illustrations.

Binding:

N/A (Digital Format)

Paper Quality:

N/A (Digital Format)

⚙️ Engine Specifications

Engine Type:

Liquid-cooled, 2-stroke, reed valve induction

Displacement:

125 cc

Bore X Stroke:

54 mm x 54.5 mm

Compression Ratio:

8.8:1 (typical, may vary slightly)

Cylinder Count:

1

Port Design:

Yamaha Power Valve System (YPVS)

Horsepower:

Approximately 35-40 HP (stock, peak power)

Torque:

Approximately 17-19 lb-ft (stock, peak torque)

Carburetor Model:

Mikuni TM38 flat-slide

Fuel Type:

Unleaded gasoline, minimum 91 octane (RON+MON)/2

Fuel Mixture:

50:1 ratio of gasoline to 2-stroke oil (specific oil recommended in manual)

Oil Type:

Yamaha 2M-CF Premium 2-Stroke Oil or equivalent API-TC, JASO FD certified oil

Ignition Type:

Digital CDI (Capacitor Discharge Ignition)

🔧 Transmission

Transmission Type:

Constant-mesh 6-speed

Clutch Type:

Wet, multi-plate

Gear Ratios:

Detailed ratios for each gear provided in manual.

Final Drive:

Chain drive, 520 pitch

Transmission Oil Type:

Yamalube 10W-30 API SE or SF or SG class (specifics in manual)

📐 Dimensions

Length:

2,135 mm (84.1 in)

Width:

825 mm (32.5 in)

Height:

1,270 mm (50.0 in)

Wheelbase:

1,475 mm (58.1 in)

Ground Clearance:

370 mm (14.6 in)

Dry Weight:

93 kg (205 lbs)

Wet Weight:

Approximately 104 kg (229 lbs) with fluids

Fuel Capacity:

8.2 liters (2.2 US gal)

Oil Capacity:

0.6 liter (0.63 US qt) for transmission

🛡️ Safety Features

Safety Classification:

Competition Off-Road Motorcycle (not street legal)

Braking System:

Hydraulic disc brakes front and rear for superior stopping power.

Suspension Design:

Advanced long-travel suspension (Kayaba USD forks, Monoshock rear) designed for impact absorption and control.

Frame Construction:

Lightweight and strong aluminum alloy frame for rigidity and handling.

💡 Technology Features

Infotainment System:

N/A (Dedicated Competition Motorcycle)

Assistance Features:

N/A (Dedicated Competition Motorcycle)

Connectivity Features:

N/A (Dedicated Competition Motorcycle)

🔧 Maintenance

Oil Change Transmission:

Every 15-20 hours of operation or annually.

Air Filter Cleaning:

After every 2-3 rides or as needed based on dust conditions.

Spark Plug Inspection:

Every 10-15 hours of operation.

Chain Lubrication Adjustment:

Every 5-7 rides or before each race.

Top End Rebuild Piston:

Recommended every 30-50 hours of hard use, or as indicated by performance decline.

Suspension Service:

Fork seal replacement every 20-30 hours; full fork/shock service every 40-60 hours.

Fork Oil Type:

Kayaba KSH-10W or equivalent, specified viscosity and level in manual.

Brake Fluid Type:

DOT 3 or DOT 4 hydraulic brake fluid.

Common Wear Items:

Clutch plates, piston rings, crank bearings, fork seals, chain and sprockets.

Engine Tuning Notes:

Proper jetting and YPVS function are critical for optimal 125cc two-stroke performance. Manual provides detailed guidance.

📚 Vehicle History

Yamaha Legacy:

Yamaha has a long-standing reputation for building high-performance, reliable two-stroke motocross bikes.

Yz125 Lineage:

The YZ125 is a cornerstone of Yamaha's motocross lineup, evolving through numerous technological advancements since its inception. The 2004 model represents a mature iteration of this successful platform.

2004 Updates:

The 2004 YZ125 received significant updates, including a revised frame, updated suspension settings, and engine tuning for improved power delivery and handling, building upon the successful '03 model.

Manufacturing Location:

Japan

Production Era:

Continuous production from the 1970s to the present, with the 2004 model being part of the 3rd major generation.

🏆 Awards & Recognition

Motocross Awards:

The YZ125 platform has historically been a favorite among motocross riders and magazines for its performance and handling, often receiving accolades in its class.