Motorcycle Segment:
Motocross Competition
Target Audience:
Enthusiasts, mechanics, owners of the 1986 Yamaha YZ125
Key Competitors Era:
Honda CR125R, Suzuki RM125, Kawasaki KX125
Vehicle Focus:
Lightweight, agile, performance-oriented motocross machine
Manual Benefit:
Enables owners to perform maintenance and repairs to factory standards, preserving performance and longevity
Engine System:
Complete engine overhaul, piston, rings, cylinder, crankshaft, bearings, seals, carburetor tuning, exhaust system
Transmission System:
Clutch assembly, gear shifting mechanism, gearbox inspection, lubrication
Chassis Suspension:
Front forks (KYB or similar), rear shock absorber (Monoshock), linkage system, swingarm maintenance
Brakes System:
Front and rear drum brake systems, brake shoe replacement, cable adjustment
Electrical System:
Ignition system, lighting, wiring diagrams, troubleshooting electrical faults
Cooling System:
Air-cooled engine design, cylinder head cooling fins
Fuel System:
Fuel tank, petcock, fuel lines, carburetor cleaning and adjustment (e.g., Mikuni VM series)
Body Frame:
Frame inspection, bodywork removal and installation, seat, fenders, fuel tank paneling
Maintenance Tasks:
Regular scheduled maintenance, oil changes, filter cleaning/replacement, spark plug replacement, chain adjustment
Troubleshooting Guides:
Diagnostic charts for common running issues, starting problems, performance degradation
Repair Procedures:
Step-by-step instructions for disassembly, repair, and reassembly of all major components
Technical Specifications:
Torque specifications for all fasteners, engine clearances, tuning data
Depth Level:
Comprehensive Workshop Level
Diagnostic Capabilities:
Detailed troubleshooting for mechanical and electrical issues
Rebuild Guidance:
Complete engine and transmission rebuild procedures
Bore X Stroke:
56.0 mm x 49.8 mm
Compression Ratio:
8.4:1 (Approximate)
Cooling System:
Air-cooled
Cylinder Configuration:
Single cylinder
Valvetrain:
Reed valve induction
Ignition System:
CDI (Capacitor Discharge Ignition)
Horsepower:
Approx. 27-29 HP (Est. for the era)
Torque:
Not explicitly stated, but strong low-mid range power delivery expected
Carburetor Model:
Mikuni VM28SS (Typical)
Fuel Type:
Unleaded gasoline with 2-stroke oil mix
Fuel Mix Ratio:
Typically 20:1 to 32:1 (Check manual for exact specification)
Overall Length:
Approx. 2160 mm (Varies slightly)
Overall Width:
Approx. 820 mm (Varies slightly)
Overall Height:
Approx. 1260 mm (Varies slightly)
Wheelbase:
Approx. 1455 mm
Dry Weight:
Approx. 97 kg
Wet Weight:
Approx. 101 kg (Includes fluids, est.)
Oil Change Frequency:
After every few rides or as specified in the manual (depending on severity of use)
Air Filter Cleaning:
Regularly, often before each riding session or after dusty conditions
Spark Plug Inspection:
Check condition and gap at regular intervals, replace as needed
Chain Tension Adjustment:
Check and adjust before each ride
Transmission Oil:
Yamalube Gear Oil SAE 10W-30 or equivalent API GL-4 (Check manual for specific PN)
2 Stroke Oil:
High-quality 2-stroke motorcycle oil, specifically for racing applications (e.g., Yamalube 2R or similar)
Brake Fluid:
DOT 3 or DOT 4 (Check manual for specific requirements, though likely not applicable to drum brakes needing fluid)
Carburetor Jetting:
Can be sensitive to altitude and temperature; requires proper jetting for optimal performance.
Clutch Wear:
Clutch plates can wear over time due to aggressive riding; periodic inspection and replacement are necessary.
Suspension Maintenance:
Front fork seals and rear shock absorber can require rebuilding or replacement due to heavy use.
Maintenance Schedule Reference
Schedule Availability:
Detailed service intervals for engine, chassis, and drivetrain are provided in the manual.
Yamaha Motorsports:
Yamaha's long-standing commitment and success in motocross racing
Yz Series Legacy:
Part of Yamaha's dominant YZ lineage, known for innovation and performance
Manufacturing Location:
Japan (Primary)
Production Period:
The YZ125 has been in continuous production since the 1970s, with the 1986 model representing a specific evolutionary step.
