Market Segment:
Performance-oriented motocross competition motorcycle.
Competitors 2007:
Honda CR125R, Kawasaki KX125, Suzuki RM125, KTM 125 SX.
Engine Type:
Liquid-cooled, 2-stroke, reed valve induction, YZ-Injected (YIP) system (though not as advanced as later models, it's a key feature for the era).
Suspension:
Kayaba Speed Sensitive System (SSS) front forks and Kayaba rear shock, offering excellent adjustability and performance.
Illustrations:
Detailed, step-by-step illustrated instructions for clarity and ease of use.
Engine System:
Engine overhaul, top-end rebuild, bottom-end assembly, lubrication, cooling, ignition, carburetion, exhaust.
Transmission System:
Clutch assembly, gearbox operation, shift mechanism, maintenance, and repair.
Chassis System:
Frame, swingarm, suspension (front forks, rear shock), steering system, bearings.
Brakes System:
Hydraulic front and rear brake systems, caliper service, master cylinder overhaul, bleeding procedures.
Electrical System:
Wiring diagrams, lighting, ignition system components, battery, troubleshooting electrical faults.
Fuel System:
Fuel tank, fuel lines, fuel filter, carburetor (Mikuni VM28SS), adjustments, and cleaning.
Wheel Tire System:
Wheel bearing maintenance, spoke tension, tire replacement, rim service.
Bodywork Fairings:
Removal and installation of body panels, fenders, seat.
Routine Maintenance:
Pre-ride inspection, oil changes, air filter cleaning, chain lubrication and adjustment, spark plug replacement, brake pad inspection.
Engine Rebuild:
Piston replacement, ring replacement, cylinder inspection, crankshaft and connecting rod service, seal and gasket replacement.
Suspension Service:
Fork oil change and seal replacement, rear shock absorber service and tuning.
Carburetor Tuning:
Jetting recommendations for various conditions, idle speed adjustment, float level adjustment.
Troubleshooting:
Diagnostic procedures for common mechanical and electrical issues.
Torque Specifications:
Detailed torque values for all critical fasteners.
Special Tools:
Identification of required special tools for specific repair procedures.
Depth:
Intermediate to Advanced. Covers complete disassembly, inspection, repair, and reassembly of all major motorcycle systems.
Engine Type:
Liquid-cooled, 2-stroke, single-cylinder
Bore X Stroke:
54 mm x 54.5 mm
Compression Ratio:
8.8:1 (nominal)
Intake System:
Reed valve
Carburetor:
Mikuni VM28SS
Lubrication:
Premix (2-stroke oil and gasoline)
Horsepower:
Approx. 35-40 HP at 11,000 RPM (estimated, varies with tuning and condition)
Torque:
Approx. 16-18 lb-ft at 10,000 RPM (estimated)
Fuel Type:
Unleaded gasoline (91 octane or higher recommended)
Fuel Tank Capacity:
8.0 liters (2.1 US gal)
Transmission Type:
Constant mesh, 6-speed
Clutch:
Wet, multiple-disc
Final Drive:
O-ring sealed chain
Gear Ratios:
1st: 2.615 (34/13), 2nd: 1.933 (29/15), 3rd: 1.556 (28/18), 4th: 1.333 (24/18), 5th: 1.143 (24/21), 6th: 1.000 (23/23) (Typical ratios, refer to manual for exact)
Transmission Oil Type:
Yamalube 2R (or equivalent API TC, JASO FD certified 2-stroke oil)
Transmission Oil Capacity:
N/A (Premix engine)
Safety Standard:
N/A (Off-road competition vehicle, not subject to street-legal safety standards).
Brakes:
Hydraulic disc brakes, front and rear for superior stopping power.
Suspension Performance:
Advanced adjustable suspension for stability and control over varied terrain.
Frame Construction:
Lightweight and rigid aluminum frame for responsive handling.
Oil Change Interval:
Refer to manual; typically every 10-15 hours of riding or annually.
Air Filter Cleaning:
After every few rides, or more frequently in dusty conditions. Refer to manual.
Spark Plug Replacement:
Every 20-30 hours of riding. Refer to manual for specific plug type and gap.
Chain Lubrication:
Before every ride, or as needed.
Chain Adjustment:
Check regularly, adjust to specified slack. Refer to manual.
Fork Oil Type:
Yamalube Fork Oil G10 or equivalent.
Fork Oil Capacity:
Refer to manual for specific amounts per fork leg (e.g., 550-600 cc, check manual).
Coolant Type:
Permanent type anti-freeze coolant (50% mixture with distilled water).
Brake Fluid Type:
DOT 4 Hydraulic Brake Fluid.
Clutch Wear:
Clutch plates can wear over time due to aggressive riding; regular inspection is recommended.
Power Valve Maintenance:
The power valve system requires regular cleaning and inspection to ensure proper operation and prevent carbon buildup.
Carburetor Jetting:
Carburetor jetting may need adjustment based on altitude, temperature, and riding style for optimal performance.
Yamaha Legacy:
Yamaha has a long and successful history in motocross racing, with the YZ line being a dominant force for decades.
Yz125 Lineage:
The YZ125 has undergone continuous development since its inception, focusing on power delivery, handling, and suspension technology.
Production Period:
The YZ125 has been in continuous production since the 1970s, with significant updates in design and technology over the years.
Manufacturing Location:
Japan (Typical for Yamaha YZ models)
