Segment:
Competitive 125cc two-stroke motocross segment, known for its agility and performance.
Competitors:
Key competitors in this era included the Honda CR125R, Suzuki RM125, and Kawasaki KX125.
Vehicle Attributes:
Renowned for its potent two-stroke engine, lightweight chassis, and excellent handling characteristics, making it a favorite for racing and aggressive trail riding.
Utility:
Provides essential information for owners and technicians to maintain, diagnose, and repair the 2003 YZ125, ensuring optimal performance and longevity.
Primary Coverage:
Complete service and repair for the 2003 Yamaha YZ125.
Specific Systems:
Engine, transmission, chassis, suspension, brakes, electrical system, fuel system, cooling system, exhaust system.
Type:
Factory Service Manual / Repair Manual
Format:
Digital Download (PDF)
Engine Assembly:
Complete engine tear-down, inspection, assembly, including top-end (cylinder, piston, rings, cylinder head), bottom-end (crankshaft, connecting rod, bearings), and power valve system.
Transmission And Drivetrain:
Gearbox disassembly, inspection, and reassembly; clutch operation and service; final drive (chain and sprockets) inspection and replacement.
Fuel And Ignition:
Carburetor disassembly, cleaning, and adjustment; fuel tank and petcock service; ignition system (CDI, spark plug, coil) testing and troubleshooting.
Suspension And Steering:
Front fork (USD KYB) disassembly, seal replacement, oil change, and adjustment; rear shock absorber (Monoshock) service and adjustment; steering system inspection and bearing replacement.
Braking System:
Front and rear hydraulic disc brake system service, including caliper overhaul, master cylinder service, bleeding procedure, and pad replacement.
Electrical System:
Wiring diagrams, lighting system, charging system (stator, flywheel), switches, and basic troubleshooting.
Scheduled Maintenance:
Detailed procedures for routine checks, lubrication points, and replacement intervals for common wear items.
Troubleshooting Guides:
Systematic diagnostic procedures for common performance issues, starting problems, and component failures.
Rebuild Procedures:
Step-by-step instructions for complete engine and component rebuilds, suitable for both amateur and professional mechanics.
Special Tools:
Identification and usage of specialized tools required for specific repair and maintenance tasks.
Diagnostic Capability:
Comprehensive diagnostics covering all major systems, including electrical troubleshooting with wiring diagrams.
Repair Scope:
Full spectrum of repairs from minor adjustments and fluid changes to complete engine and chassis overhauls.
Illustrations And Diagrams:
Abundant exploded views, part diagrams, and photographic illustrations to guide repairs.
Type:
Liquid-cooled, 2-stroke, Reed valve induction
Bore X Stroke:
54.0 mm x 54.5 mm
Compression Ratio:
8.6:1 (for 2003 model)
Power Valve System:
Yamaha Power Valve System (YPVS)
Horsepower:
Approx. 35-38 HP (estimated, varies with tuning)
Torque:
Approx. 16-18 lb-ft (estimated)
Carburetor:
Keihin PWK38S (38mm flat slide, dual-jet system)
Fuel Type:
Unleaded gasoline (91-96 octane recommended)
Fuel Oil Mix:
50:1 ratio with high-quality 2-stroke oil (refer to manual for specific recommendations)
Coolant Capacity:
Approx. 0.65 liters
Transmission Oil:
Yamalube SAE 10W-30 API SE or SF (refer to manual for precise specification and capacity)
Type:
Constant mesh 6-speed
Clutch Type:
Wet, multiple-disc
Gear Ratios:
1st: 2.615, 2nd: 1.933, 3rd: 1.470, 4th: 1.190, 5th: 1.000, 6th: 0.864 (Refer to manual for exact ratios)
Final Drive:
Chain drive (RK 520 H-ring chain recommended)
Drive Ratio:
14/50 (standard, may vary)
Length:
Approx. 83.3 inches (2115 mm)
Width:
Approx. 32.7 inches (830 mm)
Height:
Approx. 50.2 inches (1275 mm)
Wheelbase:
Approx. 57.7 inches (1465 mm)
Ground Clearance:
Approx. 13.2 inches (335 mm)
Seat Height:
Approx. 38.4 inches (975 mm)
Dry Weight:
Approx. 200 lbs (90.7 kg)
Fuel Tank Capacity:
Approx. 2.3 gallons (8.5 liters)
Oil Tank Capacity:
Not applicable (pre-mix engine)
Classification:
Motocross competition motorcycle; designed for closed-course competition use only.
Braking System:
Hydraulic disc brakes (front and rear) with high-performance calipers and rotors.
Suspension System:
Advanced KYB front forks and rear monoshock suspension for superior control and impact absorption.
Frame Construction:
Lightweight, high-strength aluminum frame for optimal rigidity and rider feedback.
Oil Change:
Every 10-15 hours of operation or annually (refer to manual for specific racing vs. trail usage).
Air Filter Cleaning:
As needed, typically every 5-10 hours or after dusty rides.
Spark Plug Replacement:
Every 20-30 hours of operation or as indicated by performance.
Top End Rebuild:
Recommended every 40-60 hours of aggressive riding or 80-100 hours of trail riding, depending on maintenance and usage.
Engine Oil Type:
Yamalube SAE 10W-30 API SE/SF or equivalent 2-stroke transmission oil.
Coolant Type:
High-quality ethylene glycol-based antifreeze coolant (50% mixture with distilled water).
Brake Fluid Type:
DOT 3 or DOT 4 brake fluid.
Carburetion Tuning:
Can be sensitive to altitude and temperature changes, requiring proper jetting adjustments for optimal performance.
Clutch Wear:
Clutch plates and springs may require periodic inspection and replacement due to the high-performance nature of the engine.
Power Valve Operation:
Carbon buildup on the YPVS can affect performance and may require periodic cleaning.
Yamaha Legacy:
Yamaha has a long and successful history in motocross, with the YZ line being a cornerstone of their performance off-road offerings for decades.
Yz125 Development:
The YZ125 has undergone continuous refinement, with significant updates in chassis design, engine performance, and suspension technology throughout its production run.
Production Years:
1974 - Present (with various updates and iterations)
Manufacturing Location:
Japan