Segment:
Motocross / Off-Road Competition
Key Competitors 2007:
KTM 125 SX, Honda CR125R, Suzuki RM125, Kawasaki KX125
Chassis Design:
Aluminum bilateral beam frame for lightweight and rigidity.
Suspension Front:
Industry-leading KYB Speed Sensitive System (SSS) forks.
Suspension Rear:
Yamaha's renowned Monoshock system with adjustable damping.
Engine Characteristics:
Strong mid-range power delivery, sharp throttle response.
Engine:
Complete engine disassembly, assembly, tuning, and troubleshooting.
Cooling System:
Radiator, water pump, hoses, coolant specifications, and bleeding procedures.
Fuel System:
Carburetor specifications, jetting, float adjustment, fuel pump (if applicable), and fuel lines.
Ignition System:
CDI unit, spark plug specifications, coil, and timing.
Transmission:
Clutch operation, gear selection, transmission disassembly, assembly, and lubrication.
Suspension:
Front forks (KYB SSS), rear shock absorber (Monoshock), suspension settings, seal replacement, and fluid changes.
Brakes:
Hydraulic front and rear disc brakes, caliper overhaul, brake pad replacement, fluid specifications, and bleeding procedures.
Electrical System:
Wiring diagrams, lighting, battery, stator, and troubleshooting electrical faults.
Chassis And Frame:
Frame inspection, steering head bearings, and general chassis components.
Bodywork:
Plastics, seat, fuel tank, and decals.
Maintenance:
Regular service checks, lubrication points, filter replacements, and pre-ride inspections.
Troubleshooting:
Step-by-step guides for diagnosing and resolving common issues.
Repair:
Detailed instructions for engine rebuilds, component replacement, and adjustments.
Assembly:
Complete reassembly procedures after disassembly.
Disassembly:
Comprehensive breakdown of all major components.
Tuning:
Carburetor jetting, suspension adjustment, and general performance tuning.
Depth:
Comprehensive (from basic maintenance to complete engine rebuilds).
Configuration:
125cc, 2-stroke, single-cylinder
Bore X Stroke:
54.0 mm x 54.5 mm
Compression Ratio:
8.6:1 (typical)
Cooling System:
Liquid-cooled
Horsepower Estimated:
35-40 HP (at crankshaft, varies with tuning)
Torque Estimated:
20-22 Nm (at crankshaft, varies with tuning)
Carburetor Model:
Keihin PWK38S (or similar 38mm flat slide)
Fuel Type:
Unleaded gasoline (91 octane or higher recommended)
Premix Ratio:
50:1 (with high-quality 2-stroke oil)
Ignition Type:
Digital CDI
Oil Change Recommendation:
Refer to manual for specific intervals based on riding conditions (e.g., every 10-15 hours of intense use for gearbox oil).
Air Filter Cleaning:
Inspect and clean/replace after every few rides, depending on dust conditions.
Spark Plug Replacement:
Consult manual; typically every 20-30 hours of use, or as needed based on performance.
Top End Rebuild:
Recommended every 40-60 hours of race use for optimal performance, though can vary.
Gearbox Oil:
Yamalube Gear Oil 10W-30 (or equivalent API GL-4 specification)
Coolant:
Yamaha-specific liquid coolant or a high-quality permanent type antifreeze (50/50 mix with distilled water).
Brake Fluid:
DOT 4 hydraulic brake fluid.
Clutch Wear:
Clutch plates can wear over time, especially with aggressive riding. Regular inspection and adjustment are recommended.
Carburetor Jetting Sensitivity:
Highly sensitive to altitude, temperature, and humidity. Proper jetting is crucial for performance.
Suspension Maintenance:
Regular fork seal and shock absorber servicing is vital for consistent handling.
Yamaha Motocross:
Yamaha has a long and successful history in motocross racing, with the YZ line being a cornerstone of their off-road performance.
Yz125 Development:
The YZ125 has consistently evolved with chassis and engine technology, aiming for competitive performance in the highly contested 125cc class.
Yz125 Production Era:
Continuous production since the 1970s.
