A blocked air supply makes the engine suck too much fuel for the air it gets. The result is a rich mixture: wasted fuel, black smoke and a carbon-fouled spark plug. Dirt is the usual cause, but things can also block the airbox inlet from outside or from inside. Check this before touching the carburettor jets.
Safety
- Engine off and cool.
- Do not store loose items under the seat if the airbox inlet is there.
Steps
- Look at the airbox inlet and any ducts. Make sure nothing is stuffed or pressed against it, such as a cloth under the seat.
- Open the airbox. Look for dust, leaves, nests and insects. Mice and wasps can build nests inside the airbox.
- Inspect the filter. - Paper: hold it to a light. If it is dark, oily, or wet, replace it. Paper that has got wet passes less air even after it dries, and a bike that has stood a long time can absorb moisture. Blowing with air cleans it only a little. - Foam or fabric: see Clean and re-oil a foam air filter.
- Check the crankcase breather. A worn engine can push oil vapour through the breather and into the airbox, and this can plug the filter. A filter on the breather hose, with the hose blocked off where it enters the airbox, may be needed. See your manual.
- Check the air tube between airbox and carburettor for cracks and splits. See Replace a hard, shrunk air tube.
- Refit the filter and cover so the seal is flat.
Check your work
- Clear airbox with a clean, dry filter.
- Plug colour improves and the exhaust smoke stops after a test ride.