A little vapour on a cold morning is normal. A plume that persists, smells sharp or appears under load is not. Note when it happens and what it smells like before you start dismantling.
Blue or grey smoke
Oil is entering the combustion chamber or the exhaust. Worn rings, valve guides and seals, a failed head gasket or an overfilled crankcase are the usual paths. A leak-down test and a wet/dry compression test help.
On a bike that has sat, oil can seep past valve guides and burn off at start-up. Persistent smoke under load is more serious.
White smoke
White smoke that smells sweet is usually coolant. A head gasket, a cracked head or a warped sealing surface can let coolant into the cylinder. A pressure test and a check for exhaust gases in the coolant confirm it.
White vapour that disappears as the engine warms is usually condensation, especially in humid weather.
Black smoke
Black smoke is unburned fuel. A rich mixture, a leaking injector, a blocked air filter or a failed sensor can cause it. A black plug and high fuel consumption support the diagnosis.
Do not lean the mixture to hide the symptom. Find out why the fuel is not burning.
Check the crankcase and breather
An overfilled engine or a blocked crankcase breather can push oil into the intake. Check the oil level correctly and inspect the breather system before assuming piston rings.
A used bike with unknown history may have the wrong oil or an additive that is burning off. Change it and recheck.
Checklist
- When the smoke appears: cold, hot, idle, under load.
- Smell: oil, coolant or fuel.
- Oil level and condition.
- Coolant level and pressure test.
- Compression and leak-down.
- Crankcase breather and airbox for oil.
- Spark plug colour and injector condition.
Questions
Is a little smoke at start-up normal?
A puff of vapour on a cold, humid morning can be normal. Persistent blue or white smoke is not.
Can a head gasket cause white smoke?
Yes. A pressure test and a check for combustion gases in the coolant are the usual confirmation.
Does black smoke mean the engine is worn?
Not necessarily. It usually means the mixture is rich. Check the air filter, injectors and sensors first.