Engine Service:
Complete engine disassembly, inspection, repair, and reassembly procedures.
Carburetor Service:
Detailed carburetor tuning, cleaning, and rebuild instructions.
Transmission Service:
Gearbox inspection, clutch service, and shift mechanism repair.
Suspension Service:
Front fork (KYB or similar) and rear monoshock (Monocross system) overhaul, seal replacement, and oil changes.
Braking System:
Front and rear brake caliper service, master cylinder overhaul, pad replacement, and bleeding procedures.
Electrical System:
Ignition system troubleshooting, wiring diagrams, lighting service, and component testing.
Cooling System:
Radiator inspection, hose replacement, and coolant specifications (though likely air-cooled for this era).
Fuel System:
Fuel tank, petcock, and fuel line inspection and replacement.
Frame And Bodywork:
Frame inspection, body panel removal/installation, and seat maintenance.
Routine Maintenance:
Detailed procedures for oil changes, air filter cleaning/replacement, spark plug service, and chain adjustment.
Troubleshooting:
Common problems, diagnostic flowcharts, and solutions for various operating issues.
Rebuilds:
Complete engine rebuild procedures, including piston, ring, and cylinder honing/replacement.
Adjustments:
Carburetor jetting, ignition timing, clutch lever free play, and brake lever adjustments.
Troubleshooting Engine:
Diagnosis of starting problems, misfiring, poor power, and overheating.
Troubleshooting Suspension:
Diagnosis of leaks, stiff operation, or bottoming out.
Diagnostics:
In-depth diagnostic steps for identifying and resolving mechanical and electrical faults.
Specifications:
Precise torque values, clearances, fluid capacities, and performance data.
Wiring Diagrams:
Comprehensive electrical system schematics with component identification.
Engine Rebuild Level:
Full engine rebuild including crankshaft, bearings, seals, and cylinder porting (if modifications are considered).
Electrical Diagnostics Level:
Advanced troubleshooting of ignition timing, coil output, and charging systems (if applicable).
Manual Scope:
Comprehensive coverage for all maintenance, repair, and diagnostic procedures.
Engine Type:
2-Stroke, Single-Cylinder, Reed Valve
Displacement:
124cc (approximate, specific bore/stroke needed)
Cooling System:
Air-cooled
Bore X Stroke Mm:
Refer to manual for exact dimensions (typically around 56mm x 50.7mm).
Horsepower Estimated:
30-35 HP (typical for class in this era)
Torque Estimated:
17-20 lb-ft (typical for class in this era)
Carburetor Model:
Mikuni VM Series (likely 34mm or 36mm)
Fuel Type:
Unleaded gasoline (minimum 90 octane recommended) mixed with 2-stroke oil.
2 Stroke Oil Ratio:
Typically 20:1 or 32:1 (manual will specify exact ratio).
Ignition System:
CDI (Capacitor Discharge Ignition)
Engine Oil Change Frequency:
As per manual recommendations, typically every 10-15 hours of riding or after major engine work.
Air Filter Cleaning:
Regularly, often every few rides, especially in dusty conditions.
Spark Plug Replacement:
Check and clean every 10-20 hours, replace as needed based on condition.
Transmission Oil Type:
Yamalube 10W-30 or equivalent API SF, SE or SD classification gear oil (specifics in manual).
Fork Oil Type:
Yamaha Fork Oil G10S or equivalent SAE 10W fork oil (specific viscosity and quantity in manual).
Brake Fluid Type:
DOT 3 or DOT 4 (refer to manual for specific recommendation).
Typical 1987 Yz125 Wear Items:
Clutch wear, piston ring wear, carburetor jetting issues due to fuel variations, suspension seal leaks.
Preventative Maintenance:
Regular engine top-end inspections (piston and rings), clutch adjustment, and suspension servicing are crucial for longevity.
Common Carb Issue:
Pilot jet clogging, main jet wear, float level adjustment drift.
Yamaha Motorcycles:
Yamaha has a long and storied history in motorsports, particularly in motocross, with a legacy of innovation and performance.
Yz Series Development:
The YZ series has consistently evolved with technological advancements, leading the pack in performance and design for decades.
Yz125 Key Advancements Pre 1987:
Introduction of Monocross rear suspension, reed valve induction, and advanced chassis designs.
Yz125 Production Era:
The YZ125 has been in continuous production since its inception, making the 1987 model a key part of its development lineage.
