When warm or hot pipes cool down moisture laden air (humidity) deposits condensation on the inside of the exhaust system. If it can't or doesn't drain out, the next time the engine is run the heat vaporizes it and it appears as white smoke (steam) exiting the system. If the engine and pipes are heated long enough, then all the water turns to vapor until depleted, whereby the white smoke ceases. If the pipes are not throughly heated and of sufficient duration, water can build up in exhaust low points with repetative heat/cool cycles. This could increase the duration of white smoke during warm ups. This occurs frequently during winter operation when the pipes are more difficult to heat completely. This assumes an air cooled engine such as the SOHC4. A liquid cooled engine can have the water jacket leak into the combustion chambers and then exit as white smoke, also. Anti-freeze makes the exhaust kind of sweet smelling. At least before asphyxia.
Unburned hydrocarbons, the result of a fuel mixture too rich, generally exit the exhaust as black smoke. It also often leaves a deposit of soot inside the exhaust system, as well. It will feel dry and powdery on the fingers.
Blue smoke, or smoke with a bluish tinge, is an indication of oil being burned and exhaust system deposits are black and oily on the fingers.
Oil Leaks-
Near each end of the CB500/CB550 cylinder bank is an oil passage providing oil to the valve train. Where the head meets the cylinder block, an o ring is used to seal this passage. With age and heat the o ring looses its seal and begins to weep. It can, but not always, get progressively worse. I've had one weeping for the last 20 years. It's anoying, for sure, but the bike just runs too well to take it apart for that minor leak. To fix it, the head will have to be renewed as well as the gaskets and o rings. You'll have to decide whether your leak is bad enough to address.
Cheers,
