If white smoke emanates from your motorcycle's exhaust system, you may need to visit an expert for repairs. The specific reasons for the smoke will differ based on the engine type, the age of the bike and its overall condition.
If your motorcycle has been sitting unused for awhile, white smoke may come from condensation build-up in the piston chambers, mufflers or any part of the exhaust system. This is normal and will not harm the engine. The smoke will cease as the engine warms.
If your motorcycle is liquid-cooled, white smoke may be a sign of engine coolant entering the cylinder chambers. This may be caused by a blown head gasket. Check to see if you have leaking fluids in addition to the white smoke.
White, greyish-white or bluish-white smoke can be a sign of burning oil in an air-cooled engine. Oil may leak into the cylinder chambers and ignite along with the normal gasoline/air mixture. Make sure that your gaskets and seals are not cracked or degraded and that you have not overfilled the oil.
Cold weather can cause the coolant to freeze and crack a cylinder wall, which will allow coolant to leak into your oil. This mixture will produce white smoke when you start the bike.
Never operate your motorcycle if you believe it is mechanically unsound, and have all repairs performed by a professional motorcycle mechanic.