Smoke coming from a motorcycle exhaust indicates the engine is not running correctly. Typical problems include fuel not being completely burnt, too much oil or coolant evaporating. If ignored, such problems can result in significant engine damage, and being able to identify the origin of the smoke, as quickly as possible, can reduce the risk of a costly repair. The colour of the smoke is the main indicator of where the problem lies: white smoke indicates coolant- or water-related issues while darker smoke is common for fuel or oil problems.
Remove and inspect the spark plugs if you see grey or blue exhaust smoke. Black soot on the spark plugs suggests that the fuel/air mixture is too rich, resulting in grey exhaust smoke. A shiny, wet, black film on the plugs is a result of excess oil, and this causes blue smoke.
Check and clean the air cleaner which may have become clogged, causing grey smoke. The air cleaner may also have worked itself loose, in which case a new one will need to be fitted. If grey smoke persists, consult the user's manual for your bike to find out how to make the fuel mixture more lean.
Inspect the piston ring and valve seals to ensure they are airtight. If you are unsure how to do this and your user's manual does not provide instructions, seek professional assistance. Failure of these seals results in burning oil, hence a blue smoke, and the seals should be replaced immediately.
Check the ambient air temperature. If it is below 10 degrees Celsius, then white smoke is perfectly normal until the engine warms up. If it continues after the engine is warm, then there is a source of excess water somewhere in the engine.
Check if there is too much oil in the oil reservoir and, if so, drain the excess oil and see if the exhaust continues to smoke. This is the most obvious cause of smoke when the engine burns oil.
Inspect the cylinder heads, seals and pistons for wear. These faults can be caused by an engine overheating and can result in a blown head gasket. This is generally not a job that can be conducted at home, and any checks/repairs should be completed by your local dealer.
If you are unsure as to what is creating the exhaust smoke, do not continue to run the motorcycle as you may cause substantial damage to the engine. Instead, seek professional assistance as soon as possible.