1978 Yamaha YZ125
Specifications and workshop reference information for the 1978 Yamaha YZ125, including engine, chassis, service data and setup details.
Note: Specs are compiled from factory data, manuals and owner feedback. Bikes vary — if yours is different, jump into the forum and share it.
General Specifications
| Engine Type | Air-cooled 2-stroke single cylinder |
| Displacement | 123 cc |
| Bore × Stroke | 56.0 × 50.0 mm |
| Compression Ratio | Approx. 7.2:1 |
| Maximum Power | Approx. 23 hp @ 8,500 rpm |
| Carburetor | Mikuni VM28 |
| Fuel Mix Ratio | Premix (approx. 20:1–32:1 depending on oil) |
| Ignition | CDI |
| Spark Plug | NGK B8ES |
| Spark Plug Gap | 0.6–0.7 mm |
| Starting System | Kickstart |
Transmission
| Transmission Type | 5-speed |
| Clutch | Wet multi-plate |
| Final Drive | Chain drive |
Dimensions
| Wheelbase | Approx. 1,350 mm |
| Ground Clearance | Approx. 280 mm |
| Fuel Tank Capacity | Approx. 7.5 litres |
Chassis
| Frame Type | Steel single cradle |
| Front Suspension | Telescopic forks (increased travel) |
| Rear Suspension | Twin shock swingarm (improved travel) |
Brakes & Wheels
| Front Brake | Drum |
| Rear Brake | Drum |
| Front Tire | 3.00-21 |
| Rear Tire | 4.00-18 |
125cc Other Specs
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