A healthy, warmed-up four-stroke engine puts out an almost invisible gas from the exhaust. Visible smoke can show that something that should not be burning inside the engine is burning, or that the mixture is off. But not all smoke is a fault: white vapour seen on a cold morning is often completely normal. In this guide we explain what smoke indicates according to its colour, when it appears and its smell, and when to wait and when to go to a workshop straight away.
Quick table: what does the colour tell you?
| Colour and situation | Possible meaning | What to do |
|---|---|---|
| Thin white vapour on a cold start, disappearing as the engine warms up | Normal condensation (water vapour) | Do nothing |
| Thick white smoke that continues when warm, sweetish smell | On a liquid-cooled engine, coolant getting into the combustion chamber | Do not ride, go to a workshop |
| Blue or grey smoke, smell of burnt oil | Oil burning: rings, cylinder, valve seals or too much oil | Check the oil level, show it to a workshop |
| Black smoke, smell of petrol, sooty exhaust tip | Rich mixture: blocked air filter, choke left on, carburettor or injection problem | Check the air filter and choke; if it continues, go to a workshop |
White smoke: often harmless, sometimes serious
When fuel burns, water vapour is also produced. In cold weather, or when the engine is first started, the exhaust is cold, so this vapour condenses and shows as a white cloud, or even as water dripping from the exhaust tip. It disappears as the engine and exhaust warm up. On motorcycles used for short distances the exhaust never warms up fully, so this vapour can be visible for longer.
The white smoke you should worry about behaves differently:
- It continues heavily even after the engine is fully warm,
- It has a sweetish, distinctive smell,
- The level in the coolant reservoir drops for no reason,
- The temperature gauge reads higher than normal.
This picture can show that on a liquid-cooled engine coolant is leaking into the combustion chamber; it is often a problem with the head gasket or the cylinder head. In this case, carrying on riding makes the damage worse; it is safer to take it to a workshop on a recovery truck. We explained how the cooling system works in the air-cooled or liquid-cooled guide. Because an air-cooled engine has no coolant, thick and persistent white smoke there has other causes, often related to oil burning.
Blue smoke: the engine is burning oil
Bluish or grey smoke and the smell of burnt oil show that oil is getting into the combustion chamber and burning along with the fuel. Possible causes:
- Worn ring or cylinder: Oil passes the piston into the combustion chamber. NGK also lists oil consumption from worn rings and cylinder walls among the causes of plug fouling. The smoke is usually most noticeable under load, while accelerating.
- Valve seals: A brief puff of blue smoke can be seen when starting after the engine has stood for a while, or when you open the throttle after a long downhill.
- Too much oil: If the crankcase is overfilled, oil can get into the breather system and from there into the intake. The oil quantity is in the manual; for example, in the Turkish manual for Honda's CB125F (SC74 type) it is 0.8 litres after draining. Always check the level by the method in the manual.
- Two-stroke engines: On a two-stroke the oil is burnt along with the fuel, so some blue smoke is normal; but if it increases, the mixture ratio or the oil pump should be checked.
Honda's manual reminds you that engine oil consumption varies with riding conditions and that the level should be checked and topped up as needed. If you check the oil level regularly, you will notice oil burning from the falling level before you see smoke. For choosing the right oil, see the engine oil guide.
Black smoke: the mixture is too rich
Black smoke shows that the engine is taking in more fuel than it can burn. The fuel does not burn completely, leaves the exhaust as soot and blackens the exhaust tip. Possible causes:
- Blocked air filter: When the air is reduced, the mixture becomes richer. The cheapest item and the first to check.
- Choke left on: On carburettor engines, if the choke is not closed after the engine has warmed up.
- Carburettor setting or float: A wrong setting or a stuck float can deliver too much fuel.
- Injection system: A faulty sensor or a leaking injector. On fuel-injected motorcycles, if the warning light is on, a workshop reads the fault code.
A rich mixture also coats the plug in soot; NGK lists a rich mixture among the causes of carbon fouling. Along with black smoke, misfiring and rising fuel consumption are also seen. We explained how to read the plug in the spark plug guide and the causes of misfiring in the misfiring guide.
To observe the smoke properly
- Start the engine cold and watch the smoke in the first minutes.
- Look again at idle once the engine has warmed up; has the vapour gone?
- Ask a helper to watch you: is there smoke while accelerating, or when you close the throttle and open it again?
- Pay attention to the smell: sweetish, burnt oil or raw petrol?
- Check the levels: are the oil and, if there is any, the coolant dropping?
- Note what you see or take a short video; it helps when you describe it to the workshop.
Exhaust smoke and buying used
When looking at a used motorcycle, it matters that you start the engine, not the seller; an engine that has been warmed up beforehand hides the smoke and the starting difficulty on a first start. Blue smoke, thick white smoke or constant black smoke on a warm engine should be decisive in whether you buy at all, not a bargaining point. The full checklist is in the used bike guide.
Before tampering with the exhaust
You cannot solve a smoke problem by changing the exhaust or removing the silencer; the problem is inside the engine. Also, the penalty for making a modification to the vehicle contrary to the regulations is 5,000 TL for the operator and a bar from traffic until the vehicle is brought into compliance; if the modification produces noise that disturbs the public, the driver is additionally fined 16,000 TL and the vehicle is barred from traffic for 30 days.
Conclusion
Thin white vapour that lasts briefly on a cold start is normal. Thick white smoke that continues once warm, together with falling coolant, is a sign of a serious problem; do not ride, go to a workshop. Blue smoke indicates oil burning and black smoke a rich mixture: first look at the oil level, the air filter and the choke, and if it does not improve take it to a workshop. Noting the colour, timing and smell makes diagnosis easier.
Frequently asked questions
Why does a motorcycle give off white smoke when cold?
Water vapour formed during combustion condenses in the cold exhaust and shows up as a white cloud. If it disappears as the engine warms up, it is normal.
Why does an engine burn oil?
Worn rings and cylinder, worn valve seals or an overfilled crankcase are the most likely causes. If blue smoke and a falling oil level are seen together, show it to a workshop.
Can I ride a motorcycle that gives off black smoke?
You can for a short time, but a rich mixture fouls the plug, raises fuel consumption and in time damages the engine. Check the air filter and choke first, and if it continues go to a workshop.
Is water dripping from the exhaust normal?
Some water dripping from the exhaust tip on a cold start comes from condensation and is normal. If it continues after the engine has warmed up and the coolant is dropping, show it to a workshop.