Segment:
Motocross Competition
Target Audience:
Motocross racers, mechanics, and performance enthusiasts
Competitors 1986:
Honda CR125R, Kawasaki KX125, Suzuki RM125
Yz125 1986 Highlights:
Known for its strong performance in its class, Yamaha's YZ series was a significant player in amateur and professional motocross racing. The 1986 model likely featured advancements in suspension and engine tuning for competitive edge.
Primary Coverage:
Complete Service, Repair, and Workshop Manual
Estimated Page Count:
300-400 pages (estimated for a comprehensive manual of this type and era)
Type:
Service Repair Workshop Manual
Format:
Digital (likely PDF, though physical copies may exist)
Engine:
Top End Rebuild (Piston, Rings, Cylinder), Bottom End Rebuild (Crankshaft, Connecting Rod, Bearings), Carburetion, Cooling System (if applicable, though likely air-cooled), Lubrication, Ignition System, Exhaust System
Transmission:
Gearbox Disassembly and Reassembly, Clutch Operation and Service, Shift Mechanism
Chassis:
Frame Inspection and Repair, Suspension (Front Fork and Rear Monoshock), Steering System, Wheel Bearings, Swingarm and Linkages
Brakes:
Front and Rear Brake System Overhaul (Hydraulic Disc Brakes Likely), Master Cylinders, Calipers, Lines, Pads
Electrical:
Ignition System Components, Lighting (if applicable), Wiring Diagrams, Troubleshooting Electrical Faults
Bodywork:
Body Panel Removal and Installation, Seat, Fuel Tank, Fairings (if applicable)
Maintenance:
Regular Service Intervals, Lubrication Points, Chain Adjustment, Air Filter Cleaning/Replacement, Spark Plug Service, Gear Oil Change
Troubleshooting:
Diagnostic Procedures for Common Issues, Symptom-Based Problem Solving
Repair:
Detailed step-by-step instructions for component disassembly, repair, and reassembly
Specifications:
Torque Specifications, Clearances, Fluid Capacities, Electrical Specifications
Engine Rebuild Stages:
Piston/ring replacement, cylinder honing, crankshaft inspection/replacement, bearing replacement, seal replacement, gasket replacement.
Carburetor Overhaul:
Disassembly, cleaning, inspection of jets, float height adjustment, reassembly.
Suspension Tuning:
Front fork oil level/viscosity adjustments, rear shock damping adjustments (where applicable), spring preload adjustments.
Electrical Diagnostics:
Ignition timing checks, stator/coil testing, wiring continuity checks, troubleshooting no-spark conditions.
Depth:
Comprehensive, covering everything from routine maintenance to complete engine and transmission overhauls and chassis component repairs.
File Type:
PDF (common for digital manuals)
Searchability:
Likely text-searchable if it's a well-produced PDF.
Printability:
Designed for printing individual sections or the entire manual.
Binding:
N/A (for digital format); Typically spiral-bound or perfect-bound for printed versions.
Paper Quality:
N/A (for digital format); Original manuals often used durable, gloss-coated paper.
Type:
Two-stroke, Single Cylinder
Displacement:
124 cc (cubic centimeters)
Bore X Stroke:
56.0 mm x 50.7 mm (typical for this model)
Cooling System:
Air-cooled
Horsepower:
Approx. 28-32 HP (estimated, peak performance figures vary)
Torque:
N/A (specific torque figures less commonly published for this era of MX bikes)
Carburetor:
Mikuni VM Series (e.g., VM28SS or similar)
Fuel Type:
Unleaded Gasoline with 2-stroke Oil Mixture
Compression Ratio:
Approx. 7.8:1 to 8.5:1 (typical for the era, may require specific octane fuel)
Spark Plug Type:
NGK BR8ES or equivalent (verify in manual)
Overall Length:
Approx. 2120 mm (83.5 in) (typical for this model)
Overall Width:
Approx. 800 mm (31.5 in) (typical for this model)
Overall Height:
Approx. 1250 mm (49.2 in) (typical for this model)
Wheelbase:
Approx. 1440 mm (56.7 in) (typical for this model)
Dry Weight:
Approx. 94 kg (207 lbs) (typical for this model)
Fuel Tank Capacity:
Approx. 8.0 Liters (2.1 US gal) (typical for this model)
Oil Change Frequency:
After every few races or X hours of riding (specifics in manual)
Air Filter Cleaning:
After every riding session or as needed based on conditions
Spark Plug Replacement:
Periodically, or when fouled/worn (as per manual recommendations)
Coolant Type:
N/A (Air-cooled)
2 Stroke Oil Mix Ratio:
Typically 20:1 or 32:1 with high-quality 2-stroke racing oil (verify in manual)
Common Wear Items:
Piston and rings (due to high operating temperatures and stress), clutch plates, wheel bearings, swingarm bearings, fork seals.
Carburetor Tuning:
Can be sensitive to altitude and temperature changes, requiring periodic jetting adjustments.
Suspension Maintenance:
Regular fork oil changes and seal replacements are crucial for optimal performance and longevity.
Yamaha Motorsport:
Yamaha has a long and storied history in motorcycle racing, particularly in motocross, dating back to the 1960s. The YZ line represents their dedication to competition.
Yz125 Development:
The YZ125 has been a staple in Yamaha's lineup, undergoing continuous development to improve power delivery, suspension performance, and handling to remain competitive in the highly contested 125cc class.
Production Period:
Ongoing production since the 1970s, with significant updates throughout the decades.
