Segment:
Performance Motocross Motorcycle
Target Audience:
Competitive motocross riders, mechanics, and serious enthusiasts
Competitors:
Honda CR125R (discontinued), KTM 125 SX, Suzuki RM125 (discontinued), Kawasaki KX125 (discontinued)
Engine Technology:
Yamaha's patented Power Valve System (YPVS) for improved power delivery across the rev range.
Suspension Technology:
KYB Speed Sensitive System (SSS) front forks offering sophisticated damping characteristics.
Handling:
Renowned for its agile chassis and predictable handling, a hallmark of the YZ line.
Engine Mechanical:
Complete engine tear-down, rebuild, component inspection, specifications, torque values
Fuel System:
Carburetor (Mikuni TMX38) tuning, cleaning, jetting, fuel delivery
Ignition System:
CDI, spark plug specifications, troubleshooting ignition faults
Cooling System:
Liquid cooling system maintenance, coolant specifications, radiator, water pump
Transmission Drivetrain:
Clutch operation, gear shifting mechanism, gearbox disassembly and assembly, lubrication
Suspension:
Front forks (KYB Speed Sensitive System) servicing, oil levels, adjustments, rear shock absorber servicing
Brakes:
Front and rear hydraulic brake system maintenance, fluid replacement, pad replacement, bleeding procedures
Electrical System:
Wiring diagrams, lighting, switches, battery maintenance, troubleshooting basic electrical issues
Chassis Frame:
Frame inspection, sub-frame, bodywork removal and installation
Wheels Tires:
Wheel bearing maintenance, tire changing, spoke tensioning
Maintenance:
Routine checks, lubrication points, filter replacements, fluid changes
Troubleshooting:
Diagnostic flowcharts for common problems, ignition, fuel, and mechanical issues
Repair:
Step-by-step instructions for component disassembly, repair, and reassembly
Rebuild:
Complete engine rebuild procedures
Diagnostics:
Fault finding and rectification methods
Depth:
Comprehensive, factory-level detail for advanced mechanics and DIY enthusiasts
Diagrams Illustrations:
Includes pictures and part diagrams for visual guidance
Engine Displacement:
125cc
Engine Configuration:
Liquid-cooled, 2-stroke, reed valve, single-cylinder
Bore X Stroke:
54 mm x 54.5 mm
Compression Ratio:
8.6:1 (with YPVS in closed position)
Carburetor:
Mikuni TMX38 (flat-slide)
Ignition:
Digital CDI with TCI (Transistor Controlled Ignition)
Oil Injection:
Yamaha Energy Induction System (Yamalube 2-stroke oil)
Horsepower:
Approximately 36-38 HP (estimated, actual figures vary with tuning)
Torque:
Approximately 20-22 lb-ft (estimated)
Fuel Type:
Unleaded gasoline (91 octane or higher recommended)
Fuel Capacity:
8.0 Liters (2.1 Gallons)
Oil Change:
Refer to manual for specific hours/miles, typically every 15-20 hours of riding for competitive use.
Air Filter Cleaning:
Recommended after every few rides, or more frequently in dusty conditions.
Spark Plug Replacement:
As per manual recommendations or when fouled/worn.
Piston Ring Replacement:
Typically every 30-50 hours for aggressive riding; consult manual for specific guidance.
Top End Rebuild:
Recommended every 50-70 hours for optimal performance, dependent on riding style.
Engine Oil Type:
2-stroke premix oil (e.g., Yamalube 2-S)
Transmission Oil Type:
Yamaha Gear Oil (or equivalent SAE 10W-30 API SE, SF, SG)
Coolant Type:
High-quality motorcycle coolant (50/50 mix of ethylene glycol and distilled water)
Common Wear Items:
Clutch plates, piston rings, spark plugs, chain and sprockets, brake pads, fork seals.
Maintenance Criticality:
Proper pre-mix ratio and clean air filter are crucial for longevity of 2-stroke engines.
Heritage:
Yamaha has a long and successful history in motocross, with the YZ line being a dominant force for decades.
Evolution:
The YZ125 has undergone continuous development, evolving from early air-cooled models to the sophisticated liquid-cooled, YPVS-equipped machines of the modern era.
Production Era:
YZ125 models have been in continuous production since the 1970s.
