An older two-stroke blows a lot of smoke, even though the oil mix or the pump setting is correct.
Is it safe to ride?
Ride with care. Do not cut back the oil to reduce the smoke before the seals are checked. Too little oil can seize the engine.
Quick checks first
- Is the pre-mix ratio or pump setting correct for the engine?
- Is the oil tank full and the pump working?
- Is the air filter clean?
Diagnose it step by step
1Is the oil mixed or injected at the rate in the manual?
- No: correct it. See Mix two-stroke pre-mix oil and petrol or Test the output of a two-stroke oil pump.
- Yes: go to step 2.
2Does the gearbox oil level rise, or does the oil look thin?
- Yes: oil from the gearbox is passing the crankshaft seal into the engine. This adds to the smoke. See Remove and fit an engine oil seal.
- No: go to step 3.
3Is the engine worn?
- See Test engine compression. Low compression means the engine does not burn the fuel and oil as well as it once did.
Causes, most likely first
| Cause | How to confirm | Fix |
|---|---|---|
| Engine worn, fuel and oil burned poorly | Low compression | Rebuild the top end |
| Old hard crankshaft seals | Gearbox oil level rises, oil thin | Remove and fit an engine oil seal |
| Too much oil in the mix or from the pump | Smoke at idle, fouled plugs | Correct the ratio or pump setting |
Old hard seals can let gearbox oil into the engine, so part of the oil being burned may not come from the pump. Cutting back the mix to stop the smoke leaves the engine short of oil if the seals then soften and seal again. The fix is new crankshaft seals. See also Blue or grey smoke from the exhaust.