Segment:
High-performance two-stroke motocross motorcycle.
Target Audience:
Motocross racers, off-road enthusiasts, and owners of the Yamaha YZ125.
Competitors:
KTM 125 SX, Honda CR125R, Kawasaki KX125, Suzuki RM125 (during relevant model years).
Vehicle Focus:
Lightweight chassis, powerful two-stroke engine, advanced suspension technology (KYB SSS).
Manual Value Proposition:
Enables owners to perform detailed maintenance and repairs, saving time and money, and ensuring optimal performance and longevity of the YZ125.
Engine:
Complete engine disassembly, assembly, tuning, troubleshooting, internal components, cooling system, lubrication system.
Transmission:
Clutch, gearbox, shift mechanism, lubrication, troubleshooting.
Fuel System:
Carburetor (Keihin PWK), fuel lines, fuel tank, fuel pump (if applicable), mixture adjustments.
Ignition System:
CDI, spark plug, ignition timing, troubleshooting ignition faults.
Exhaust System:
Expansion chamber, silencer, maintenance, repair.
Suspension:
Front forks (KYB SSS - Speed Sensitive System), rear shock absorber (Monoshock with linkage), adjustments, rebuilds, oil changes, seal replacement.
Brakes:
Front and rear hydraulic disc brakes, caliper maintenance, pad replacement, fluid bleeding, master cylinder service.
Wheels And Tires:
Wheel bearing replacement, spoke tensioning, tire changes, rim maintenance.
Electrical System:
Wiring diagrams, lighting (headlight, taillight), battery maintenance, stator, rectifier/regulator (if applicable).
Frame And Bodywork:
Chassis, subframe, plastic body panels, seat, handlebars, controls, removal and installation.
Maintenance:
Routine checks, oil changes, filter cleaning/replacement, chain adjustment, spark plug service, air filter service, coolant flush (if liquid-cooled).
Troubleshooting:
Diagnostic procedures for common starting issues, running problems, electrical faults, and performance issues.
Repair:
Detailed step-by-step instructions for component repair, rebuilding, and replacement.
Disassembly Assembly:
Complete guidance on taking apart and reassembling all major vehicle systems and components.
Tune Ups:
Carburetor tuning, ignition timing adjustments, engine break-in procedures.
Depth:
In-depth technical information suitable for experienced home mechanics and professional technicians. Includes torque specifications, clearances, and detailed diagrams.
Type:
Liquid-cooled, 125cc, two-stroke, reed-valve induction
Bore X Stroke:
54.0 mm x 54.5 mm
Compression Ratio:
8.8:1 (typical for this era)
Carburetor:
Keihin PWK38S (38mm) with Power Jet
Horsepower:
Approx. 37-39 HP @ 11,000 RPM (typical for model years)
Torque:
Approx. 18-19 lb-ft @ 10,000 RPM (typical for model years)
Fuel Type:
Unleaded gasoline (minimum 91 octane recommended)
Premix Ratio:
50:1 (Yamalube 2R or equivalent 2-stroke oil recommended)
Features:
N/A (Motocross motorcycle, no infotainment system)
Features:
N/A (Motocross motorcycle, no driver assistance systems)
Features:
N/A (Motocross motorcycle, no connectivity features)
Powervalve:
Yamaha Power Valve System (YPVS) for optimized power delivery across the RPM range.
Suspension Technology:
KYB Speed Sensitive System (SSS) front forks (introduced around this era), offering advanced damping characteristics and adjustability.
Oil Change Engine:
Every 15-20 hours of riding, or annually. Refer to manual for specific recommendation.
Air Filter Cleaning:
After every few rides, or as needed based on riding conditions. Essential for engine longevity.
Chain Lubrication Adjustment:
Before every ride or every 5-10 hours. Crucial for drivetrain performance and wear.
Spark Plug Inspection:
Every 10-15 hours of riding.
Engine Oil Type:
Yamaha Yamalube 2R or equivalent high-quality 2-stroke motorcycle oil (premix).
Transmission Oil Type:
Yamalube 10W-30 or equivalent API SG/SH/SJ transmission oil.
Coolant Type:
High-quality pre-mixed ethylene glycol based coolant with corrosion inhibitors.
Brake Fluid Type:
DOT 4 hydraulic brake fluid.
Common Wear Items:
Clutch plates, piston rings, spark plugs, air filters, chain and sprockets, brake pads.
Engine Performance Degradation:
Can occur due to improper premix ratio, worn piston/rings, or clogged exhaust/carburetor.
Suspension Maintenance:
Regular fork seal maintenance and shock absorber service are critical for optimal performance.
