Market Segment:
Performance Motocross / Off-Road Motorcycle
Target Audience:
Owners, mechanics, and enthusiasts of the 1991 Yamaha YZ125 looking for detailed technical information.
Contemporary Competitors:
Honda CR125R, Suzuki RM125, Kawasaki KX125.
Vehicle Era Focus:
Represents a significant period in 125cc two-stroke motocross development.
Manual Value Proposition:
Essential resource for maintaining and restoring the peak performance of a classic YZ125.
Engine:
Complete engine disassembly, assembly, tuning, troubleshooting, and component specifications.
Transmission:
Clutch, gearbox, and final drive maintenance, repair, and adjustment.
Suspension:
Front fork (telescopic), rear shock absorber (monoshock), linkage, and adjustment procedures.
Brakes:
Front and rear brake system maintenance, bleeding, caliper overhaul, and pad replacement.
Electrical:
Ignition system, lighting, wiring diagrams, charging system, and troubleshooting electrical faults.
Cooling:
Radiator, hoses, and coolant maintenance (if liquid-cooled, though 1991 YZ125 is typically air-cooled).
Fuel System:
Carburetor disassembly, cleaning, tuning, fuel tank, and fuel line maintenance.
Frame And Bodywork:
Frame inspection, seat, fenders, handlebars, and controls adjustment/replacement.
Wheels And Tires:
Wheel bearing maintenance, spoke tensioning, tire mounting, and rim specifications.
Preventive Maintenance:
Regular service checks, lubrication points, and filter replacement schedules.
Troubleshooting:
Diagnostic guides for common problems, symptom analysis, and systematic fault finding.
Disassembly And Assembly:
Detailed, step-by-step instructions for component removal and reinstallation.
Tuning And Adjustment:
Carburetor jetting, ignition timing, chain adjustment, and suspension setup.
Rebuild Procedures:
Complete engine rebuilds, including piston, cylinder, crankshaft, and gearbox.
Torque Specifications:
Precise torque values for all critical fasteners and bolts.
Depth:
In-depth, professional-level detail suitable for experienced mechanics and dedicated enthusiasts.
Illustrations:
Clear, step-by-step illustrations and diagrams to guide procedures.
Diagnostics:
Advanced diagnostic information for identifying and rectifying complex issues.
Engine Type:
Liquid-Cooled, 2-Stroke, Reed Valve
Bore X Stroke:
56.0 mm x 50.0 mm
Compression Ratio:
8.4:1 (Typical, check manual for exact figure)
Carburetor:
Mikuni VM28SS (Single Carburetor)
Ignition:
CDI (Capacitor Discharge Ignition)
Horsepower:
Approximately 35-37 HP (at peak RPM, actual figures vary with tuning and condition)
Torque:
Approximately 21-23 Nm (at peak RPM, actual figures vary)
Fuel Type:
Unleaded Gasoline (91-98 Octane Recommended)
Fuel Mixture:
Pre-mix (Gasoline to 2-stroke oil ratio, typically 20:1 or 25:1, check manual)
Fuel Tank Capacity:
Approximately 8.5 Liters (2.2 US gallons)
Overall Length:
Approximately 2135 mm (84.1 in)
Overall Width:
Approximately 810 mm (31.9 in)
Overall Height:
Approximately 1240 mm (48.8 in)
Seat Height:
Approximately 955 mm (37.6 in)
Wheelbase:
Approximately 1445 mm (56.9 in)
Dry Weight:
Approximately 93 kg (205 lbs)
Wet Weight:
Approximately 99 kg (218 lbs) (with all fluids)
Oil Change Frequency:
Transmission oil: Check and change per operating hours or mileage, as specified in the manual (e.g., every 10-15 hours of hard use).
Air Filter Cleaning:
Clean and re-oil after every few rides or in dusty conditions.
Spark Plug Inspection:
Inspect and clean/replace at regular intervals (e.g., every 20-30 hours of use).
Top End Rebuild:
Piston and ring replacement recommended typically between 40-80 hours of racing use, depending on riding style and tuning.
Transmission Oil:
Yamalube 10W-30 or equivalent API GL-4 gear oil. Capacity: Approximately 0.8 Liters. (Verify in manual).
Coolant:
If liquid-cooled, use a 50/50 mix of ethylene glycol antifreeze and distilled water. (1991 YZ125 is typically air-cooled, but manual will confirm).
Fork Oil:
Yamaha Specially Spec Fork Oil G10 or equivalent. (Viscosity and quantity specified in manual).
Power Valve:
Potential for carbon buildup or sticking of the exhaust power valve mechanism, requiring cleaning and lubrication.
Clutch Wear:
Clutch plates can wear over time due to heavy use; torque and adjustment are critical.
Carburetor Tuning:
Sensitivity to jetting changes with altitude, temperature, and exhaust modifications.
Suspension Seals:
Fork seal leaks are common with age and use, requiring replacement.
Yamaha Motocross:
Yamaha has a long and successful history in motocross racing, with the YZ line being a cornerstone.
Yz125 Development:
The 1991 model represented an evolution of Yamaha's 125cc motocross platform, with continuous updates in suspension, engine performance, and chassis design.
Yz125 Production Years:
The YZ125 has been in continuous production for Yamaha for many decades, starting in the early 1970s.
