Segment:
Motocross Competition
Target Audience:
Motocross racers, enthusiasts, mechanics
Competitors:
KTM 125 EXC, Honda CR125R, Suzuki RM125, Kawasaki KX125
Engine Type:
2-stroke, liquid-cooled
Suspension Front:
Upside-down telescopic fork (Kayaba)
Suspension Rear:
Monoshock with linkage (Monocross)
Braking Front:
Hydraulic disc
Braking Rear:
Hydraulic disc
Primary Coverage:
Full service, repair, and maintenance
Secondary Coverage:
Troubleshooting, diagnostics, specifications
Type:
Service Repair Workshop Manual
Engine:
Top end, bottom end, cooling, lubrication, ignition, fuel, exhaust
Transmission:
Clutch, gearbox, shifting mechanism, final drive
Chassis:
Frame, suspension (front forks, rear shock), steering, wheels, brakes
Electrical:
Ignition system, lighting, wiring diagrams, switches, CDI unit
Fuel System:
Carburetor (Mikuni VM28SS), fuel tank, fuel lines
Maintenance:
Oil changes, filter replacement, chain adjustment, spark plug checks
Repair:
Engine rebuild, carburetor tuning, suspension overhaul, brake system service
Troubleshooting:
Diagnosis of common issues, fault finding charts
Disassembly Assembly:
Complete step-by-step guides for all major components
Depth:
Comprehensive, suitable for professional mechanics and experienced DIY enthusiasts
Diagnostics:
Detailed diagnostic procedures and specifications
Engine Type:
2-stroke, reed valve induction, liquid-cooled
Bore X Stroke:
54.0 mm x 54.5 mm
Compression Ratio:
8.8:1 (nominal)
Carburetor:
Mikuni VM28SS
Ignition System:
CDI (Capacitor Discharge Ignition)
Spark Plug:
NGK BR8ES or equivalent
Premix Ratio:
50:1 (Yamalube 2R or equivalent premium 2-stroke oil)
Horsepower Estimated:
Approx. 34-36 hp @ 11,000 RPM
Torque Estimated:
Approx. 17-18 lb-ft @ 10,000 RPM
Fuel Type:
Unleaded gasoline (minimum 91 octane recommended)
Fuel Tank Capacity:
7.2 Liters (1.9 US Gallons)
Transmission Type:
6-speed constant mesh
Clutch Type:
Wet, multi-disc
Gear Ratios:
Manual will detail specific gear ratios (e.g., 1st: 2.769, 2nd: 2.166, 3rd: 1.750, 4th: 1.400, 5th: 1.130, 6th: 0.916)
Transmission Oil Type:
Yamalube Gear Oil 10W-30 or equivalent API GL-4
Transmission Oil Capacity:
Approx. 0.8 Liters (0.85 US Quarts)
Safety Ratings:
N/A (Competition Motorcycle - Not intended for road use, no regulatory safety ratings apply)
Braking System:
Powerful hydraulic disc brakes front and rear for precise stopping power.
Suspension Performance:
High-quality suspension provides stability and control over rough terrain.
Ergonomics:
Designed for optimal rider positioning and control during demanding riding conditions.
Oil Change Engine:
As per manual recommendations, typically every 10-15 hours of riding or annually.
Air Filter Cleaning:
After every few rides, or as dictated by riding conditions.
Spark Plug Check:
Every 5-10 hours of riding.
Chain Lubrication Adjustment:
Before every ride, adjust tension as per manual.
Coolant:
50/50 mix of distilled water and permanent antifreeze (ethylene glycol based).
Brake Fluid:
DOT 3 or DOT 4 brake fluid.
Engine Wear:
As with all 2-stroke engines, regular top-end rebuilds (piston, rings) are necessary for optimal performance based on usage hours.
Clutch Slippage:
Can occur with aggressive riding or if clutch plates are worn, requiring adjustment or replacement.
Carburetor Tuning:
Requires periodic cleaning and adjustment for optimal jetting based on altitude and temperature.
Yamaha Motorsports:
Yamaha has a long and storied history in motorsports, particularly in motocross, with the YZ line being a cornerstone of their off-road racing success.
Yz125 Line:
The YZ125 has been a dominant force in the 125cc class for decades, undergoing continuous refinement to improve performance, handling, and reliability.
1999 Updates:
The 1999 model saw refinements to the engine and chassis, including revised suspension settings and minor engine tuning for improved power delivery.
Manufacturing Location:
Japan (Primary)
Production Years:
Ongoing since the 1970s
