Role:
Entry-level to intermediate motocross competition and recreational riding.
Competitors 1991:
Honda CR125R, Kawasaki KX125, Suzuki RM125.
Engine Type:
Liquid-cooled, 2-stroke, reed valve induction.
Suspension:
Monocross rear suspension, telescopic front forks.
Braking:
Hydraulic front and rear disc brakes.
Transmission:
6-speed constant mesh.
Primary Coverage:
Comprehensive service, repair, and maintenance for the 1991 Yamaha YZ125.
Secondary Coverage:
Includes diagnostic procedures, troubleshooting, and technical specifications.
Type:
Service Repair Workshop Manual
Format:
Digital (likely PDF)
Engine:
Top-end, bottom-end, cooling system, lubrication, ignition, fuel delivery, exhaust.
Transmission:
Clutch, gearbox, shift mechanism, lubrication.
Suspension:
Front forks (KYB or similar), rear shock absorber (Monocross system), linkages, seals, and adjustment procedures.
Brakes:
Front and rear hydraulic disc brakes, caliper overhaul, fluid replacement, bleeding procedures.
Electrical:
Ignition system, lighting, wiring diagrams, component testing.
Chassis:
Frame, swingarm, wheels, hubs, steering, and bodywork.
Maintenance:
Regular service intervals, oil changes, filter replacements, chain adjustment, spark plug service.
Troubleshooting:
Common issues, diagnostic charts, symptom-based problem solving.
Repair:
Detailed step-by-step instructions for component disassembly, repair, and reassembly.
Rebuilds:
Engine top-end and bottom-end rebuilds, carburetor overhaul, suspension component rebuilds.
Specifications:
Torque values, clearances, fluid capacities, electrical specifications.
Depth:
Comprehensive. Covers everything from basic adjustments to complete engine and chassis overhauls.
Engine Type:
Liquid-cooled, 2-stroke, single-cylinder
Bore X Stroke:
57.0 mm x 48.8 mm
Carburetor:
Mikuni VM28SS
Ignition:
CDI (Capacitor Discharge Ignition)
Air Cleaner:
Urethane foam element
Horsepower Estimate:
Approx. 32-35 HP @ 10,500 RPM
Torque Estimate:
Approx. 16-18 lb-ft @ 9,000 RPM
Fuel Type:
Unleaded gasoline (91 octane or higher recommended)
Fuel Mixture:
Pre-mix: 20:1 ratio (Yamalube 2 or equivalent)
Fuel Tank Capacity:
8.0 Liters (2.1 US gallons)
Infotainment System:
N/A (This is a dedicated motocross bike, no infotainment features).
Driver Assistance Features:
N/A (No electronic rider aids or driver assistance features).
Connectivity Options:
N/A (No electronic connectivity features).
Oil Change Engine:
Every 10-15 hours of riding or annually. Consult manual for precise intervals based on usage.
Air Filter Cleaning:
After every few rides, or as needed based on dust conditions.
Spark Plug Replacement:
Every 20-30 hours of riding or annually.
Chain Lubrication Adjustment:
Before every ride and check slack regularly.
Brake Fluid Flush:
Every 12 months or as per manual recommendations.
Engine Oil:
2-stroke oil (Yamalube 2 or equivalent, 20:1 mix)
Transmission Oil:
API GL-4, SAE 80W gear oil
Coolant:
Permanent type coolant (50% ethylene glycol and 50% distilled water)
Brake Fluid:
DOT 3 or DOT 4 hydraulic brake fluid
Common Wear Items:
Piston rings, clutch plates, swingarm bearings, fork seals, carburetor jets.
Two Stroke Considerations:
Proper pre-mix ratio is crucial for engine longevity. Overheating can lead to seized pistons.
Yamaha Motorsports:
Yamaha has a long and storied history in motorcycle racing, particularly in motocross, dating back to the 1960s.
Yz Lineage:
The YZ125 is part of Yamaha's iconic YZ motocross lineup, which has seen continuous development and innovation.
Yz125 1991 Context:
The 1991 model represented an evolution of the YZ platform, incorporating learnings from previous generations and race feedback.
Manufacturing Location:
Japan (Primary)
Production Period:
The YZ125 has been in continuous production for many decades, with the 1991 model being a significant iteration.
