White smoke from the exhaust

Burning gasoline naturally produces water vapor. When the engine and exhaust line are cold, especially in cool or humid weather, this vapor condenses on contact with the pipe's cold walls and comes out as a plume of white smoke, sometimes with a few drops of water at the end of the muffler. This is a perfectly normal physical phenomenon, the same as the "steam" you see coming from cars on a winter morning. As the exhaust warms up, the water stops condensing and the smoke disappears, generally within two to five minutes. No repair is needed. The signs confirming this is harmless condensation: the smoke is white and thin (not dense or oily), it has no sweet smell but a normal exhaust-gas smell, it fades once the engine is warm, and above all the coolant level stays stable. This phenomenon is more pronounced on short winter rides, since the pipe never fully dries out, and water can accumulate in the muffler (hence sometimes slight internal corrosion of exhausts on bikes that only do short trips). If you recognize this scenario, you can ride with peace of mind: it's not serious.

The head gasket seals the joint between the head and the cylinder block; it separates three circuits that must never communicate: the combustion chamber, the cooling circuit (fluid), and the oil circuit. When it degrades — often following overheating that warped the sealing surface, incorrect torque, or simply fatigue and mileage — a passage forms between the coolant circuit and the combustion chamber. Fluid then enters the cylinder and burns: the result is dense, persistent white smoke that doesn't disappear when hot, with that characteristic sweet glycol smell. Several accompanying signs help confirm it: the coolant level drops with no visible ground leak, the expansion tank can bubble (combustion gases passing into the water circuit), a beige mayonnaise-like emulsion appears under the oil filler cap and on the dipstick (water and oil mixed), and the engine can run abnormally hot. This is a serious failure: continuing to ride risks engine damage (hydraulic lock, seizure, a bent rod). The repair requires opening the engine, removing the head, checking its flatness (and machining it if warped), and reinstalling to spec with a new gasket: a major job reserved for a professional (expert-only).

Repair to be left to a professional.

Water can get into the fuel system in several ways: condensation in a tank left half-empty for a long time (humid air condenses on the cold walls), poor-quality or spoiled gasoline, a fuel cap that's no longer sealed and lets rain in, or a corroded tank that weeps. Water, being denser than gasoline, settles at the bottom of the tank and the carburetor float bowl. When the engine draws it in, it doesn't burn like gasoline: it vaporizes and produces whitish smoke, along with misfiring, hiccups, an engine that stumbles or stalls, especially at idle or when reopening the throttle. On a carbureted 4-stroke, water typically collects in the float bowl and disrupts the fuel supply. This cause is less common than simple exhaust condensation, but it's real, particularly after a long period of storage, on a bike stored all winter with a half-full tank, or after filling up at a questionable station. Diagnosis is accessible: drain the carburetor float bowl via its drain screw and observe what comes out (water is distinguishable from gasoline by its color and non-flammability), and if needed drain the questionable fuel. A job within reach of a careful home mechanic, with no engine disassembly needed.