What Causes a Motorcycle To Smoke?

The color of your exhaust smoke holds the answer.

suzuki exhaust, sukuzi intruder, motorcycle exhaust problems
Smoking Suzuki.Julia LaPalme

Q: 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? - Jeff Moore via email

A: You didn't say what color the smoke is, which can provide an important clue to what's wrong. Black or gray smoke often indicates a clogged air filter or too-rich jetting; you'd see a reduction in mileage and power. Blue smoke is usually caused by oil getting past the piston rings during acceleration or by the valve-guide seals on decel, eventually resulting in low oil level in the crankcase. White smoke means coolant is finding its way into the combustion chamber, which can cause overheating. A bad head gasket is probably the culprit.

To narrow down the problem do a leakdown test; you’ll need a special tool for this. Warm up the engine then put the piston at 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 leakdown 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.

Got a question you want answered? Send it to mcmail@bonniercorp.com.