Market Segment:
Competitive motocross racing
Key Competitors:
Honda CR125R, Kawasaki KX125, Suzuki RM125
Vehicle Philosophy:
Lightweight, agile, powerful two-stroke engine for performance-oriented riders
Target Audience:
Yamaha YZ 125 owners, mechanics, enthusiasts seeking in-depth repair and maintenance knowledge
Coverage Scope:
Complete teardown, rebuild, maintenance, troubleshooting
Estimated Page Count:
300-500
Type:
Factory Repair Service Manual
Format:
Instant Download (Digital PDF)
Engine:
Top-end rebuild (piston, rings, cylinder, head), bottom-end rebuild (crankshaft, connecting rod, bearings), carburetor overhaul, cooling system, ignition system
Transmission:
Clutch assembly, gearbox components, shift mechanism, lubrication
Chassis:
Frame inspection and repair, suspension (forks and shock absorber) disassembly, rebuild, and adjustment, wheel bearing replacement, steering system
Electrical:
Ignition system wiring, lighting (if applicable), troubleshooting electrical faults
Brakes:
Hydraulic brake system inspection, fluid replacement, caliper and master cylinder service, pad replacement
Fuel System:
Fuel tank, fuel lines, petcock, carburetor (detailed)
Maintenance:
Routine service checks, oil changes, filter cleaning/replacement, chain adjustment, spark plug replacement, air filter cleaning
Troubleshooting:
Diagnostic charts for common problems, performance issues, starting difficulties, electrical faults
Repair:
Complete disassembly and reassembly instructions for all major components
Rebuild:
Detailed steps for engine top-end and bottom-end rebuilds, suspension rebuilds
Depth Level:
Comprehensive factory-level detail, suitable for professional mechanics and experienced DIY enthusiasts
Engine Family:
YZ Series Two-Stroke
Bore X Stroke:
54.0 mm x 54.5 mm
Compression Ratio:
8.1:1 (approx.)
Cylinder Design:
Single-cylinder, liquid-cooled, reed valve induction, Yamaha Power Valve System (YPVS)
Horsepower Estimate:
35-40 HP (estimated at crank)
Torque Estimate:
18-20 lb-ft (estimated at crank)
Carburetor Type:
Keihin PWK38S (38mm slide valve)
Fuel Type:
Unleaded gasoline (91 octane or higher recommended)
Fuel Oil Mixture:
50:1 ratio with high-quality 2-stroke pre-mix oil
Ignition Type:
CDI (Capacitor Discharge Ignition)
Type:
6-speed manual transmission
Clutch Type:
Wet, multi-disc, cable-operated
Final Drive:
Chain drive (RK or similar, 520 pitch)
Transmission Oil Type:
Yamalube 2R (or equivalent API GL-4, SAE 80W transmission oil)
Overall Length:
2135 mm (84.1 in)
Overall Width:
825 mm (32.5 in)
Overall Height:
1270 mm (50.0 in)
Seat Height:
970 mm (38.2 in)
Wheelbase:
1455 mm (57.3 in)
Dry Weight:
94 kg (207 lbs)
Wet Weight:
100 kg (220 lbs) (approx., with fluids)
Fuel Tank Capacity:
8.0 L (2.1 US gal)
Front Suspension Type:
Upside-down telescopic fork
Front Fork Travel:
300 mm (11.8 in)
Rear Suspension Type:
Monocross swingarm, gas/oil-damped, adjustable preload, compression, and rebound damping
Rear Shock Travel:
320 mm (12.6 in)
Front Wheel Size:
21 x 1.60
Rear Wheel Size:
19 x 1.85
Front Tire Size:
80/100-21 M/C 51M
Rear Tire Size:
100/90-19 M/C 57M
Wheel Bolt Pattern:
N/A (spoked wheels, hub mounting)
Oil Change Interval:
Every 10-15 hours of riding (or as specified in manual for transmission oil)
Air Filter Cleaning:
After every few rides, or more frequently in dusty conditions
Spark Plug Replacement:
Every 20-30 hours of riding (or as needed)
Top End Rebuild Interval:
Every 50-100 hours of riding, depending on usage and maintenance
Coolant Type:
High-quality ethylene glycol-based coolant, 50% solution with distilled water
Fork Oil Type:
Yamaha Suspension Oil (or equivalent premium quality fork oil, e.g., Bel-Ray, Maxima, Motul) with specified viscosity (e.g., 5W, 10W)
Rear Shock Oil:
Typically nitrogen-charged, oil is specific to the shock unit (refer to manual for details)
Common Wear Items:
Piston rings, clutch plates, wheel bearings, fork seals, chain and sprockets
Engine Performance Notes:
Two-stroke engines require diligent maintenance of the power valve system (YPVS) for optimal performance and longevity. Carburetor tuning is critical for different altitudes and conditions.
Manufacturer History:
Yamaha Motor Company, established 1955, renowned for innovation in motorcycles, marine products, and audio equipment.
Yz Series History:
The YZ series has a long and successful lineage in motocross racing, with continuous development in chassis, suspension, and engine technology since its inception.
Manufacturing Location:
Japan (typically)
