Category:
Performance Motocross Motorcycle
Target Audience:
Motocross racers, off-road enthusiasts, and Yamaha YZ125 owners seeking to maintain or repair their machine.
Rivals:
Competed against models like the Honda CR125R, Kawasaki KX125, and Suzuki RM125 of the same era.
Performance Focus:
Renowned for its potent two-stroke engine and agile handling, a staple in the 125cc class.
Essential Tool:
Crucial for owners and technicians to perform accurate maintenance, diagnostics, and repairs, ensuring optimal performance and longevity.
Primary Coverage:
Complete service, repair, and maintenance procedures for the 1999 Yamaha YZ125.
Secondary Coverage:
Troubleshooting, diagnostics, component removal/installation, specifications, and diagrams.
Type:
Service Manual / Workshop Manual / Repair Manual
Engine:
Top-end (cylinder, piston, rings), bottom-end (crankshaft, connecting rod, bearings), lubrication, cooling, and exhaust systems.
Transmission:
Clutch, gearbox (gear selection, bearings, shafts), and final drive (chain, sprockets).
Fuel System:
Carburetor (disassembly, cleaning, adjustment, jetting), fuel tank, fuel lines, and fuel petcock.
Ignition System:
CDI unit, ignition coil, spark plug, wiring, and timing.
Suspension:
Front forks (disassembly, oil change, seal replacement, damping adjustments), rear shock absorber (rebuild, spring preload).
Brakes:
Front and rear hydraulic disc brakes (caliper service, pad replacement, fluid flush, master cylinder).
Electrical System:
Wiring diagrams, lighting, battery, charging system (if applicable for diagnostics), and switches.
Cooling System:
Radiator, coolant hoses, and water pump (if applicable).
Frame And Bodywork:
Frame inspection, handlebars, levers, footpegs, seat, fuel tank, and body panels.
Maintenance:
Routine checks, oil changes, filter replacements, chain adjustment, spark plug replacement, air filter cleaning/replacement.
Troubleshooting:
Diagnostic flowcharts and common problem identification for starting, running, and electrical issues.
Repair:
Detailed instructions for component disassembly, cleaning, inspection, repair, and reassembly of all major systems.
Overhaul:
Complete engine and transmission overhaul procedures.
Specifications:
Torque values, clearances, fluid capacities, and tolerances for all components.
Wiring Diagrams:
Comprehensive electrical system wiring schematics.
Tool Requirements:
Identification of special tools required for specific procedures.
Depth:
Comprehensive, covering basic maintenance to full engine and transmission rebuilds. Suitable for both home mechanics and professional technicians.
Type:
Liquid-cooled, 2-stroke, reed valve inducted
Bore X Stroke:
54.0 mm x 54.5 mm
Compression Ratio:
8.7:1 (nominal, subject to variations)
Cylinder Head:
Aluminum alloy
Piston:
Aluminum alloy, two-ring design
Horsepower:
Approximately 33-35 HP (subject to tuning and measurement conditions)
Torque:
Approximately 17-18 lb-ft (subject to tuning and measurement conditions)
Carburetor Model:
Mikuni VM28SS (likely specification, manual will confirm)
Fuel Type:
Unleaded gasoline with appropriate octane rating (e.g., 91+)
Fuel Mixture:
Requires 2-stroke engine oil mixed with gasoline (typically 20:1 or 32:1 ratio, manual will specify)
Type:
Premixed fuel/oil or automatic injection system (manual will detail specific design)
Oil Change:
Refer to manual for specific hours/miles, but typically after every few riding sessions or specific race events.
Air Filter Cleaning:
Frequent cleaning recommended, often after every 1-2 rides, especially in dusty conditions.
Spark Plug Replacement:
Refer to manual; typically every 20-40 hours of riding or as needed based on performance.
Chain And Sprockets:
Regular cleaning, lubrication, and tension adjustment; replacement based on wear.
Top End Rebuild:
Recommended based on hours of use (e.g., 30-60 hours for piston/rings, 60-100 hours for cylinder/crank bearings), manual will provide detailed guidelines.
Engine Oil Mix:
Yamaha recommended 2-stroke synthetic oil or equivalent, typically mixed at 20:1 or 32:1 ratio (manual to specify exact ratio and oil type).
Transmission Oil:
Yamaha Gear Oil (or equivalent SAE 10W-30 API SE, SF, or SG classification) - Capacity: 0.70 L (0.74 US qt)
Coolant:
Permanent type antifreeze mixed with distilled water (50:50 ratio recommended) - Capacity: 1.20 L (1.27 US qt)
Brake Fluid:
DOT 3 or DOT 4 hydraulic brake fluid (manual to specify exact type)
Carburetor Clogging:
Common in older carbureted systems due to fuel quality or debris; requires regular cleaning.
Clutch Wear:
Clutch plates can wear over time, leading to slipping; requires inspection and replacement as needed.
Suspension Seals:
Fork seals and shock seals can degrade, leading to leaks and loss of damping performance.
Power Valve Operation:
The power valve system, if equipped, can accumulate carbon and require cleaning/adjustment for optimal performance.
Yamaha Legacy:
Yamaha has a long and storied history in motorcycle racing, particularly in motocross, with numerous championships and innovations.
Yz Series Development:
The YZ125 has been a cornerstone of Yamaha's motocross lineup for decades, evolving significantly with each generation to incorporate advancements in engine technology, suspension, and chassis design.
1999 Model Year:
The 1999 YZ125 was part of a generation that saw continuous refinement, often focusing on improving power delivery, suspension performance, and reducing weight.
Manufacturing Origin:
Japan (typically)
