Positioning Statement:
The definitive resource for Yamaha YZ125 owners and technicians seeking factory-accurate repair and maintenance information.
Comparison To Aftermarket:
Vastly superior to aftermarket guides like Haynes or Clymer, offering a level of detail and accuracy only found in OEM factory publications.
Key Selling Points:
Factory direct information, comprehensive coverage, detailed diagrams, essential for accurate diagnostics and repairs.
Value To Owner:
Enables owners to perform complex repairs and maintenance with confidence, saving on labor costs and ensuring optimal performance.
Engine Overhaul:
Complete disassembly, inspection, repair, and reassembly of the 2-stroke engine.
Fuel System:
Carburetor tuning, jetting, float bowl adjustment, fuel line maintenance, fuel tank inspection.
Cooling System:
Radiator maintenance, coolant flush and fill, water pump seal replacement.
Ignition System:
CDI unit testing, spark plug specifications, stator and coil troubleshooting.
Transmission And Drivetrain:
Clutch inspection and replacement, gearbox disassembly and reassembly, chain and sprocket maintenance.
Suspension Front:
Fork disassembly, seal replacement, oil level and type specifications, damper adjustments.
Suspension Rear:
Shock absorber overhaul, linkage bearing maintenance, spring preload adjustment.
Brakes:
Hydraulic brake system bleeding, caliper rebuild, pad replacement, rotor inspection.
Electrical System:
Wiring diagrams, lighting system troubleshooting, battery maintenance, horn and indicator checks.
Frame And Bodywork:
Chassis inspection, swingarm maintenance, plastic panel removal and installation, seat replacement.
Wheels And Tires:
Spoke tensioning, rim inspection, tire pressure recommendations, bearing maintenance.
Maintenance Procedures:
Regular service intervals, pre-ride checks, post-race maintenance.
Troubleshooting Guide:
Diagnostic flowcharts for common problems and their solutions.
Repair Instructions:
Step-by-step guides for all mechanical and electrical components.
Disassembly Reassembly:
Detailed instructions for component removal and installation.
Torque Specifications:
Precise torque values for all fasteners and critical bolts.
Depth Of Information:
Factory level detail, suitable for complete engine rebuilds and advanced diagnostics.
Diagrams And Illustrations:
Numerous exploded views, schematics, and detailed illustrations.
Special Tools:
Identification of Yamaha-specific special tools required for certain procedures.
Engine Type:
Liquid-cooled, 2-stroke, reed valve induction
Bore X Stroke:
54.0 mm x 54.5 mm (typical for this era, exact may vary slightly)
Compression Ratio:
8.4:1 (typical, may vary by sub-model)
Horsepower:
Approx. 35-40 HP at peak RPM (estimated, specific dyno figures vary)
Torque:
Approx. 17-19 lb-ft at peak RPM (estimated, specific dyno figures vary)
Carburetor Model:
Mikuni VM28SS (typical for this era, verify specific year)
Fuel Type:
Unleaded gasoline with 2-stroke oil pre-mix (typically 20:1 or 32:1 depending on oil quality and use)
Oil Type Premix:
High-quality 2-stroke engine oil meeting JASO FD or ISO-L-EGD standards (manual will specify exact recommendation)
Transmission Type:
6-speed constant mesh
Optional Gear Ratios:
Not applicable for this model year.
Clutch Type:
Wet, multi-plate, manual operation
Transmission Oil Type:
Yamalube 2R or equivalent SAE 10W30 API SE or higher (verify specific year and manual)
Transmission Oil Capacity:
Approx. 0.65 - 0.75 Liters (verify specific year and manual)
Overall Length:
Approx. 80.7 inches (2050 mm) (verify specific year)
Overall Width:
Approx. 32.3 inches (820 mm) (verify specific year)
Overall Height:
Approx. 50.4 inches (1280 mm) (verify specific year)
Dry Weight:
Approx. 205 lbs (93 kg) (verify specific year)
Wet Weight:
Approx. 215-220 lbs (98-100 kg) (with all fluids, verify specific year)
Fuel Tank Capacity:
Approx. 2.1 gallons (8.0 Liters) (verify specific year)
Safety Ratings:
Not applicable to off-road vehicles in the same manner as street-legal vehicles. Focus is on rider protection and vehicle stability.
Active Safety Features:
Responsive braking system, stable chassis geometry, predictable power delivery.
Passive Safety Features:
Durable frame construction, effective suspension to absorb impacts, rider protection is paramount via aftermarket gear.
Oil Change Interval:
Transmission oil should be changed every 10-15 hours of riding or annually, whichever comes first (refer to manual for exact interval).
Air Filter Cleaning:
Clean and re-oil air filter every 5-10 hours of riding, or more frequently in dusty conditions.
Spark Plug Replacement:
Replace spark plug as per manual recommendations, typically every 20-30 hours of riding, or when fouled.
Top End Rebuild:
Piston and ring replacement recommended every 40-60 hours of hard use (refer to manual for specific wear limits).
Coolant Type:
Mixture of ethylene glycol-based coolant and distilled water (typically 50/50) (refer to manual for exact type).
Fork Oil Type And Level:
Specific suspension fluid (e.g., Yamalube Fork Oil 0W-20 or equivalent) with precise oil level specified in manual.
Common Wear Items:
Clutch plates, piston rings, crank bearings, swingarm/linkage bearings, fork seals, wheel bearings.
Typical Problems:
Carburetor jetting sensitivity to altitude and temperature, occasional water pump seal leaks, clutch fade with aggressive riding.
Yamaha Motorsports Legacy:
Yamaha has a long and storied history in motocross, with the YZ line being a cornerstone of their competitive off-road efforts for decades.
Yz125 Development:
The YZ125 has seen continuous development, with Yamaha constantly refining engine performance, suspension technology, and chassis design to remain competitive in the highly contested 125cc class.
Production Period:
The YZ125 has been in continuous production since the early 1970s, with significant updates occurring throughout its lifespan.
Motocross Competitiveness:
The YZ125 has consistently been a competitive platform in its class, earning accolades for its handling, powerband, and reliability within the motocross community.
Industry Reviews:
Period reviews from motorcycle publications often praised its agility and strong mid-range power, making it a favorite among riders.
Oem Certification:
This manual is a Yamaha factory publication, representing the highest level of official technical information.
