What Causes a Motorcycle To Smoke?

If you start up your motorcycle and notice smoke coming from the exhaust pipe, don’t worry. While it does mean that something is wrong with your motorcycle, it might be a catastrophic situation. Turn off your motorcycle and we’ll walk you through what each kind of smoke means and the route of repair that you should seek out in this MC Garage.

“My Suzuki Intruder is an occasional smoker. Most of the time it's fine, but every once in a while I'll glance back and it'll look like I'm James Bond and I just pressed the smoke-screen button. It seems like it happens more when the bike is hot, but it doesn't seem to matter if I'm accelerating or decelerating, which I vaguely remember a mechanic friend of mine saying would tell me something about the pistons or valves being the problem.

The bike is older, fully uncorked with a pipe, high-flow air filter, and jet kit. I ride it hard and it has about 60,000 miles on it, so I’m wondering if a valve job might be in order. Think that’ll fix my bike’s smoking habit?”

The color of the smoke is an important clue to what’s wrong. Black or gray smoke often indicates a clogged air filter or your bike is running too rich. If you have a motorcycle with a carburetor, you need to adjust the jetting. If your motorcycle is fuel injected, the fuel return line might be clogged or something is wrong with your injectors. You’d also notice a reduction in gas mileage and power.

Blue smoke is usually caused by oil getting past the piston rings and burning in the combustion chamber. If you notice blue smoke during acceleration or on deceleration, check your engine oil level. If it is low, then you need to replace your piston rings and inspect the cylinder walls for any damage or blemishes.

White smoke means coolant is finding its way into the combustion chamber, which can cause overheating. If your bike has been running hotter than usual or running with the radiator fans on all of the time, check your coolant level. Is it less than full? What about your oil? If your oil looks like a latte, then you have a coolant leak. A bad head gasket is probably the culprit and needs to be replaced.

To narrow down the problem do a leak down test; you’ll need a special tool for this. Warm up the engine then put the piston at the top dead center on the compression stroke so both valves are closed. Pump air into the cylinder through the spark-plug hole and measure the leak down with the attached gauge. If the leakage is higher than normal according to the shop manual, listen for a hissing sound at the end of the exhaust pipe caused by a leaking exhaust valve; hissing inside the airbox means it’s an intake valve. Worn piston rings can be heard through the oil-filler hole in the engine or oil tank, and a bad head gasket sometimes bubbles the coolant. While you’re at it check the spark plugs to make sure the engine isn’t running so lean it’s overheating and damaging the rings or valves.