Black smoke is unburned fuel. The engine is getting too much fuel for the air it breathes (a rich mixture). Common causes are a blocked air filter, a choke left on or stuck, a carburettor float that lets fuel overflow, or on fuel-injected bikes a leaking injector or a faulty sensor. A rich engine wastes fuel, fouls spark plugs, smells of petrol, and over time can thin the engine oil and damage a catalytic converter.
Is it safe to ride?
Yes with care for a short distance. Stop if fuel is dripping, the oil level is rising, or the engine warning light is flashing.
Quick checks first
- Is the choke lever or knob fully off?
- When was the air filter last cleaned or changed?
- Is fuel dripping from the carburettor overflow pipe?
- Is the engine warning light (MIL) on, on a fuel-injected bike?
Diagnose it step by step
1Confirm the colour with Diagnose exhaust smoke by colour. Does a white card held near the outlet get dry black soot?
- Yes, dry soot: rich mixture. Go to step 2.
- Oily wet spots: this is oil. Go to Blue or grey smoke from the exhaust.
2Is the choke fully off, and does the engine still smoke when warm?
- Choke on or stuck: free it with Check the choke or starter valve. See Choke is stuck on or will not work.
- Choke off: go to step 3.
3Remove and look at the air filter with Clean or replace the air filter. Is it dirty, oily or wet?
- Yes: clean or replace it. Never ride with the filter removed. Retest.
- No: go to step 4.
4Carburettor bike: is fuel overflowing from the overflow pipe, or does the engine flood when parked?
- Yes: go to Fuel pours out of the carburettor overflow pipe and Check the carburettor float and needle valve.
- No: the jets or air passages may be worn or wrong. Do Clean the carburettor. Do not guess jet sizes.
5Fuel-injected bike: is the warning light on, or are there stored codes? Read them with Read engine fault codes (fuel injection).
- Code for oxygen sensor, temperature sensor or MAP sensor: go to Check the engine sensors (TPS, MAP, temperature).
- No code: the K-Light manual notes that an oxygen sensor reading high when fully warm points to a leaking injector. Go to Check the fuel injector.
6. Read the plug with Read a spark plug. Black and dry confirms rich running. A weak spark can also leave fuel unburned; see Spark plug is black and sooty.
7. Check the oil for a petrol smell. If it smells of fuel, see Oil smells of petrol or the level rises and change the oil after the fault is fixed.
Causes, most likely first
| Cause | How to confirm | Fix |
|---|---|---|
| Choke left on or stuck | Lever position, smoke when warm | Check the choke or starter valve |
| Blocked air filter | Dirty element | Clean or replace the air filter |
| Carburettor float or needle valve | Overflow, flooding | Check the carburettor float and needle valve |
| Dirty or worn carburettor | Black plug, no overflow | Clean the carburettor |
| EFI sensor fault | Fault code | Check the engine sensors (TPS, MAP, temperature) |
| Leaking injector | Rich with no code | Check the fuel injector |