Category:
Motocross Competition Motorcycle
Era Context:
Representing the peak of 125cc two-stroke motocross technology in the late 1990s.
Key Rivals:
Honda CR125R, Kawasaki KX125, Suzuki RM125
Engine Type:
High-revving, liquid-cooled, two-stroke single-cylinder
Suspension Technology:
Advanced front forks (e.g., Kayaba or Showa) and monoshock rear suspension designed for aggressive off-road riding.
Chassis Design:
Lightweight and rigid frame optimized for handling and agility.
Engine:
Complete engine overhaul, disassembly, assembly, inspection, and tuning
Transmission:
Gearbox disassembly, assembly, inspection, clutch operation, and maintenance
Fuel System:
Carburetor disassembly, cleaning, adjustment, fuel tank, fuel lines, and fuel pump (if applicable)
Ignition System:
CDI unit, spark plug, ignition coil, wiring, and troubleshooting
Cooling System:
Radiator, hoses, coolant, water pump, and cooling fan operation
Suspension Front:
Fork disassembly, oil changes, seal replacement, spring inspection, and damping adjustments
Suspension Rear:
Monoshock disassembly, linkage, spring preload adjustment, damping settings, and rebuild procedures
Brakes Front:
Caliper service, pad replacement, disc inspection, master cylinder maintenance, and bleeding procedures
Brakes Rear:
Caliper service, pad replacement, disc inspection, master cylinder maintenance, and bleeding procedures
Electrical System:
Wiring diagrams, lighting, battery, charging system, and component testing
Chassis And Frame:
Frame inspection, headstock bearings, swingarm inspection, and bearing replacement
Wheels And Tires:
Wheel bearing service, spoke tensioning, tire mounting/demounting, and pressure recommendations
Bodywork And Fairings:
Removal and installation of plastics, seat, and other body panels
Maintenance:
Scheduled maintenance, routine checks, lubrication, and filter replacements
Troubleshooting:
Diagnostic procedures for common problems and performance issues
Repair:
Detailed repair instructions for all major components and systems
Disassembly Assembly:
Step-by-step guides for taking apart and reassembling various vehicle sections
Torque Specifications:
Precise torque values for all critical fasteners
Wiring Diagrams:
Complete and detailed electrical schematics
Depth:
Comprehensive, covering from basic maintenance to full engine rebuilds and advanced diagnostics.
Displacement:
124 cc (7.58 cubic inches)
Configuration:
Single-cylinder, two-stroke
Bore X Stroke:
54.0 mm x 54.5 mm (2.13 in x 2.15 in)
Cooling System:
Liquid-cooled
Valvetrain:
Reed valve induction
Horsepower:
Approximately 35-38 HP (specific figures vary by dyno and tuning)
Torque:
Approximately 18-20 lb-ft (specific figures vary)
Carburetor Model:
Mikuni VM28SS (typical for this era)
Fuel Type:
Unleaded gasoline with two-stroke oil pre-mix (50:1 or manufacturer recommended)
Overall Length:
Approximately 81.1 inches (2060 mm)
Overall Width:
Approximately 32.3 inches (820 mm)
Overall Height:
Approximately 49.8 inches (1265 mm)
Wheelbase:
Approximately 57.7 inches (1465 mm)
Dry Weight:
Approximately 203 lbs (92 kg)
Wet Weight:
Approximately 213 lbs (96.6 kg)
Fuel Capacity:
Approximately 2.1 gallons (8 liters)
Oil Change Engine:
Every 10-15 hours of riding, or as specified for break-in and rebuild.
Air Filter Cleaning:
Every 3-5 hours of riding, or more frequently in dusty conditions.
Spark Plug Inspection:
Every 5-10 hours of riding, or as needed based on performance.
Chain Lubrication:
Every 5-10 hours of riding, or after washing/heavy riding.
Fork Oil:
Yamaha recommended fork oil or equivalent (e.g., Bel-Ray, Motul) with specific viscosity (e.g., 5W or 10W) and level detailed in manual.
Coolant:
Mixture of distilled water and permanent antifreeze (e.g., ethylene glycol based) with corrosion inhibitors.
Transmission Oil:
Yamaha Gear Oil or equivalent high-quality 4-stroke motorcycle transmission oil (e.g., SAE 80 or SAE 10W-30).
Power Valve Sticking:
Carbon buildup can cause the power valve to stick, affecting performance. Requires regular cleaning.
Clutch Wear:
Clutch plates can wear over time, especially with aggressive riding. Manual provides inspection and replacement procedures.
Suspension Seals:
Fork and shock seals are wear items and can leak over time, requiring replacement. Manual details seal replacement.
Yamaha Motorsports:
Yamaha has a long and successful history in motocross, particularly with its YZ line.
Yz125 Line:
The YZ125 has been a dominant force in the 125cc class for decades, with continuous development and updates.
1999 Updates:
While specific year-to-year changes are best confirmed in detailed service bulletins, the 1999 model typically featured refinements in suspension, engine tuning, and chassis for improved performance.
Manufacturing Origin:
Typically manufactured in Japan by Yamaha Motor Company.
