Category:
Motocross Competition Motorcycle
Target User:
Motocross Racers, Off-Road Enthusiasts, Mechanics
Competitors 125Cc Class:
KTM 125 EXC, Honda CR125R, Suzuki RM125, Kawasaki KX125
Engine Type:
Liquid-Cooled, 2-Stroke, Reed Valve
Suspension Front:
Upside-down (USD) Cartridge Forks
Suspension Rear:
Monocross Suspension with Linkage
Braking Front:
Hydraulic Disc Brake
Engine Systems:
Top End (Cylinder, Piston, Rings, Head), Bottom End (Crankshaft, Connecting Rod, Crankcases), Cooling System (Radiators, Hoses), Lubrication System (Oil Pump, Passages), Fuel System (Carburetor, Intake), Ignition System (CDI, Spark Plug), Exhaust System
Drivetrain Systems:
Transmission (Gearbox Internals, Shifting Mechanism), Clutch (Wet Multi-Plate, Operation, Adjustment), Drive Chain and Sprockets
Chassis Systems:
Frame, Suspension (Front Forks - Damping, Rebuild; Rear Shock - Rebuild, Linkage), Steering (Headstock Bearings), Wheels and Tires (Spoke Tension, Rim Truing), Brakes (Hydraulic Front Disc, Rear Drum - Pad Replacement, Fluid Flush)
Electrical Systems:
Wiring Harness, Lighting (Headlight, Taillight, Brake Light), Ignition Switch, Kill Switch, CDI Unit, Coil, Voltage Regulator (if applicable)
Maintenance Procedures:
Routine Inspections, Oil Changes, Air Filter Cleaning/Replacement, Spark Plug Replacement, Chain Lubrication and Adjustment, Brake Bleeding, Cable Adjustments
Repair Procedures:
Engine Top-End Rebuild (Piston, Rings), Engine Bottom-End Overhaul (Bearings, Seals), Carburetor Cleaning and Re-jetting, Suspension Fork Service and Seal Replacement, Rear Shock Rebuild, Brake System Overhaul, Clutch Rebuild, Wheel Bearing Replacement, Steering Head Bearing Service
Troubleshooting Guides:
Diagnostic flowcharts for common running issues, electrical faults, and performance problems.
Depth:
Comprehensive, covering everything from basic adjustments to full engine and suspension rebuilds. Includes specifications for torque values, clearances, and fluid types.
Engine Type:
Liquid-Cooled, 2-Stroke, Reed Valve Induction
Bore X Stroke:
56.0 mm x 50.7 mm
Compression Ratio:
8.5:1 (for normal fuel)
Carburetor:
Keihin PWK38 (38mm) Single
Horsepower:
Approx. 35-40 HP (at crankshaft, estimated for the era)
Torque:
Not commonly specified in public domain for this model, focus is on peak power.
Fuel Type:
Unleaded gasoline with 2-stroke oil mix (typically 20:1 or 32:1 ratio)
Fuel Tank Capacity:
8.0 Liters (2.1 US gal)
Overall Length:
Approx. 2,130 mm (83.9 in)
Overall Width:
Approx. 800 mm (31.5 in)
Overall Height:
Approx. 1,250 mm (49.2 in)
Wheelbase:
Approx. 1,455 mm (57.3 in)
Ground Clearance:
Approx. 350 mm (13.8 in)
Dry Weight:
Approx. 94 kg (207.3 lbs)
Wet Weight:
Approx. 99 kg (218.3 lbs) (with fluids, estimate)
Seat Height:
Approx. 950 mm (37.4 in)
Safety Classification:
Off-Road Competition Vehicle - Not intended for public road use. No traditional safety ratings apply.
Braking System:
Front Hydraulic Disc, Rear Drum for stopping power.
Suspension Performance:
Advanced suspension systems designed for control and handling on rough terrain.
Chassis Design:
Robust steel frame designed to withstand impacts and stresses of motocross.
Oil Change Frequency:
Recommended every 10-15 hours of riding or annually, whichever comes first. Check manual for precise guidance based on usage.
Air Filter Cleaning:
Clean after every few rides, especially in dusty conditions. Inspect for damage.
Spark Plug Replacement:
Inspect every 10-15 hours, replace as needed. Specific plug type detailed in manual.
Chain Adjustment:
Check and adjust tension before every ride, and lubricate regularly.
Engine Oil Type:
Yamalube 2-stroke oil or equivalent JASO FD/FC, ISO-L-EGD certified 2-stroke engine oil.
Coolant Type:
Permanent Antifreeze (Ethylene Glycol based) with distilled water, typically a 50:50 mix. Refer to manual for specific Yamaha coolant or equivalent.
Brake Fluid Type:
DOT 4 hydraulic brake fluid.
Transmission Oil Specification:
Yamaha 4-stroke motorcycle gear oil or SAE 80W API GL-4.
Common Wear Items:
Piston rings, clutch plates, wheel bearings, steering head bearings, fork seals, exhaust packing.
Carburetor Tuning:
Can be sensitive to altitude and temperature changes, requiring occasional re-jetting.
Power Valve Operation:
Can accumulate carbon and require cleaning for optimal performance.
Manufacturer Founding:
1887 (Musical Instruments), 1955 (Motorcycles)
Motocross Legacy:
Yamaha has a long and storied history in professional motocross, with numerous championships and innovations in the sport.
Yz Series Introduction:
The YZ line has been a cornerstone of Yamaha's off-road offerings for decades, continuously evolving in technology and performance.
1990S Yz125 Focus:
The 1990s saw significant advancements, including the move to larger front forks (upside-down), improved power delivery, and refined chassis dynamics.
Production Region:
Japan (primarily)
