Market Segment:
High-performance 125cc two-stroke motocross bike.
Competitors:
KTM 125 SX, Honda CR125R (previous generation), Suzuki RM125 (previous generation), Kawasaki KX125 (previous generation).
Yz125 Reputation:
Renowned for its lightweight chassis, nimble handling, and potent, rev-happy two-stroke engine.
Manual Value Proposition:
Essential resource for owners and mechanics to maintain, repair, and optimize the performance of the 2012 YZ125.
Engine Mechanical:
Complete tear down and rebuild of the two-stroke engine.
Fuel System:
Carburetor tuning, fuel lines, fuel tank maintenance.
Ignition System:
CDI unit, spark plug specifications, testing procedures.
Cooling System:
Radiator maintenance, coolant specifications (if applicable for coolant-cooled models).
Transmission:
Clutch operation, gearbox disassembly and reassembly, lubrication.
Suspension:
Front forks (Kayaba), rear shock absorber (Monoshock), adjustment, service, and rebuild.
Brakes:
Front and rear hydraulic disc brake systems, fluid types, bleeding, pad replacement.
Electrical System:
Wiring diagrams, lighting, battery maintenance, troubleshooting.
Chassis And Frame:
Frame inspection, steering head bearing maintenance, swingarm lubrication.
Wheels And Tires:
Tire pressure recommendations, wheel bearing service, spoke tensioning.
Routine Maintenance:
Oil changes, filter cleaning/replacement, chain adjustment, spark plug inspection.
Troubleshooting:
Diagnostic procedures for common issues, symptom-based troubleshooting guides.
Major Repairs:
Engine rebuilds, transmission overhauls, suspension component servicing.
Technical Specifications:
Torque specifications for all critical fasteners, engine clearances, fluid capacities.
Diagnostic Checks:
Compression testing, ignition system checks, electrical system diagnostics.
Assembly Disassembly:
Step-by-step instructions for complete bike disassembly and reassembly.
Part Identification:
Detailed part diagrams for all major assemblies.
Depth:
Comprehensive, covering from basic maintenance to complete engine and chassis overhauls.
Engine Configuration:
Single-cylinder, liquid-cooled, reed-valve inducted, two-stroke.
Bore X Stroke:
54.0 mm x 54.5 mm
Compression Ratio:
8.6:1 (with pump gas), 10.2:1 (with race gas) - *Refer to manual for precise specifications*
Horsepower:
Approx. 34-36 HP (at crankshaft) - *Refer to manual for official figures*
Torque:
Approx. 17-18 lb-ft (at crankshaft) - *Refer to manual for official figures*
Carburetor:
Keihin PWK38S (38mm) with Power Jet
Fuel Type:
Unleaded gasoline (91 octane or higher recommended), requires 2-stroke oil pre-mix.
Oil Mixture Ratio:
50:1 (Yamalube 2R or equivalent 2-stroke oil recommended) - *Always verify in manual*
Ignition:
Digital CDI with TCI (Transistor Controlled Ignition)
Transmission Oil Type:
Yamalube Gear Oil 10W-40 or equivalent API GL-4
Transmission Oil Capacity:
0.75 Liters (0.79 US qt)
Safety Focus:
Designed for closed-course competition, not street legal. Safety relies on rider skill and proper protective gear.
Brakes:
Powerful hydraulic disc brakes front and rear for superior stopping power.
Suspension:
High-performance Kayaba (KYB) SSS (Speed Sensitive System) front forks and Monoshock rear suspension for optimal control and absorption.
Chassis Design:
Lightweight aluminum frame for agility and rider confidence.
Features:
N/A (Motocross bike, no infotainment systems).
Features:
N/A (Motocross bike, no electronic driver assistance systems).
Features:
N/A (Motocross bike, no connectivity features).
Spark Plug Inspection:
Every 10-15 hours of riding, or as needed based on performance.
Air Filter Cleaning:
After every ride, or more frequently in dusty conditions.
Chain Lubrication And Adjustment:
Every 5-10 hours of riding, or as needed.
Transmission Oil Change:
Every 15-20 hours of riding, or annually.
Top End Rebuild:
Typically every 40-60 hours of riding for optimal performance, depending on riding style and conditions. Refer to manual for specific piston/ring wear limits.
Suspension Service:
Every 20-30 hours of riding for fork seals and oil, and every 40-50 hours for full damper service.
Coolant:
50/50 mix of Yamalube Anti-Freeze (or equivalent) and distilled water.
Clutch Wear:
High-performance usage can lead to clutch plate wear over time; manual details inspection and replacement.
Power Valve Operation:
Carbon buildup can affect power valve function; manual provides cleaning and adjustment procedures.
Engine Maintenance Importance:
Two-stroke engines require diligent maintenance, particularly regarding lubrication and the air filtration system, to ensure longevity and performance.
Yamaha Motocross Legacy:
Yamaha has a long and storied history in motocross, consistently producing competitive and innovative machinery.
Yz125 Development:
The YZ125 has undergone continuous refinement, focusing on engine power delivery, chassis geometry, and suspension performance.
Yz125 Production Years:
Continuously produced since the 1970s, with significant updates throughout its lifespan.
