Excessive exhaust smoke can be caused by a number of things
A smoking exhaust isn't necessarily a terminal problem, but it should be sorted out sooner rather than later.
Motorcycle exhausts are supposed to be clean and smoke free: at least, that's what years of emissions regulations have attempted to create! But the internal combustion engine is not infallible and, for it to operate correctly, a whole bunch of different elements have to work in harmony. Sometimes they just don't and that's when problems can appear.
Unless you are lucky enough to own a two-stroke-engined motorcycle - an off-road bike, for example - there should be no smoke coming out of your exhaust pipe. The reason a two-stroke engine smokes is because the lubricating oil is mixed with the fuel/air and burnt in the combustion chamber, producing a lot of smoke. There are a few reasons why a four-stroke engine would be putting out smoke.
The main reason here is faulty piston rings. The purpose of piston rings is to seal the gap between the piston and the cylinder wall so that the combustion of fuel and air doesn't escape into the crankcase. That causes loss of power as not all of the force of the explosion goes into forcing the piston down.
One of the rings is called an oil-scraper ring. This prevents oil, which has been flung up into the bore to lubricate it, from travelling up into the combustion chamber. When it does, it gets burnt along with the fuel and air and produces smoke. If the oil-scraper isn't doing its job, that's what happens. Oil can also enter the combustion chamber through worn valve guides, with the same results. The only solution is an engine rebuild.
Burning oil is not necessarily detrimental to the health of the engine, unless you fail to keep an eye on the oil level and let it drop so that there's not enough lubricant to do its job properly.
Talking of oil, another reason for a smoky engine - as opposed to a smoky exhaust - is oil leaking out of the engine and being burnt off by a hot surface. The only cure for that is to stop the leak, which could mean a partial or whole strip down to replace gaskets, o-rings, etc.
The smoke from burning oil is usually blue or grey. Black smoke from the exhaust can be the product of too rich a fuel/air mixture. The easy way to tell this is by removing the spark plugs and looking at the tip of the spark plug - the part that projects into the cylinder. If it is sooty black or shiny black then the mixture is too rich, which means it is burning too much petrol. The cause of this could be a clogged air filter, meaning not enough air is getting in to mix with the fuel. Or it could be a faulty fuel injector or carburettor. Either way, you should get it seen to sooner rather than later as it's costing you money by burning too much fuel.
White smoke from the exhaust can mean that water is getting into the oil and then the combustion chamber. The water comes from the engine coolant and, if it is getting into the combustion chamber, then that could mean a blown head gasket or a crack somewhere in the block. This is serious and you really shouldn't run your bike until the problem has been solved. Like many problems, they can lead to even more damage: losing coolant could cause the engine to overheat and warp the cylinder head or even seize. You don't need to be told how dangerous that could be.
The upshot of all this is that the engine should not be putting out any smoke at all from the exhaust pipe. OK, early in the morning on start up, if the temperature is cold, then condensation in the exhaust system will come out as steam but that should disappear in a few minutes, at most.
If smoke continues to come out, then preventive maintenance is needed. In some cases, there would have been nothing you could do to prevent the start of an engine smoking, especially if it is high mileage. In other cases, regular checking of bolts, pipe connections, oil levels and air filter condition can help prevent issues.
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Harry has been writing and talking about motorcycles for 15 years, although he's been riding them for 45 years! After a long career in music, he turned his hand to writing and television work, concentrating on his passion for all things petrol-powered. Harry has written for all major publications in South Africa, both print and digital and produced and presented his own TV show called, imaginatively, The Bike Show, for seven years. He held the position of editor of South Africa's largest circulation motorcycling magazine before devoting his time to freelance writing on motoring and motorcycling. Born and raised in England, he has lived in South Africa with his family since 2002. Harry has owned examples of Triumph, Norton, BSA, MV Agusta, Honda, BMW, Ducati, Harley Davidson, Kawasaki and Moto Morini motorcycles. He regrets selling all of them.