Blue smoke from the exhaust

Bluish smoke — sometimes gray and a bit oily — comes out of the exhaust. In almost every case, this means a small amount of engine oil has gotten into the combustion chamber and is burning along with the fuel mixture: oil produces this characteristic blue tint, while excess fuel would give you black smoke, and coolant would give you white. Telling your engine type apart matters a lot here. On a two-stroke, oil is deliberately mixed with the fuel (manual mix) or metered by a pump (separate lubrication) to lubricate the bottom end: a light haze of blue smoke, especially when cold or when you roll on the throttle, is completely normal and nothing to worry about. On a four-stroke, oil and fuel are entirely separate systems: any persistent blue smoke is abnormal and points to internal wear. Signs that go along with real oil consumption: the level dropping between oil changes, an acrid, oily smell at the exhaust, a black and wet spark plug electrode, oily deposits inside the muffler. On a 4-stroke, when the smoke shows up strongly points the diagnosis in a direction: if it only appears at startup and then clears once the engine is warm, it's usually the valve stem seals; if it's there all the time and gets worse under acceleration or when you back off the throttle after a long downhill, it's more likely the piston rings. Urgency threshold: as long as you keep an eye on the oil level and top it up, you can keep riding, but consumption above roughly 0.5 L per 1,000 km calls for a prompt engine diagnosis to avoid fouling the catalytic converter and, eventually, engine damage.