Black smoke from the exhaust

Black, dry, thick smoke that smells strongly of unburned fuel comes out of the exhaust. It almost always means the same thing: the engine is getting too much fuel relative to air, the mixture is 'too rich,' and the excess fuel doesn't fully burn — it comes out as black soot. This is the exact opposite of blue smoke (oil) or white smoke (water or coolant). Whether your bike has a carburetor or fuel injection matters a lot for this diagnosis. On a carbureted bike, richness comes from a choke left on, a clogged air filter, a float bowl level that's too high (float or needle valve), or jets sized wrong. On a fuel-injected bike, it's a sensor lying to the ECU (engine temperature sensor, intake air pressure sensor, oxygen/lambda sensor) that pushes it to inject too much fuel. On a 2-stroke, slightly oilier, bluish smoke can mix in with the black due to the premix oil. Accompanying signs: a strong smell of fuel, a black and dry sooty spark plug, hard hot starts, an unstable idle, higher fuel consumption, power loss, and sometimes popping on deceleration. Urgency threshold: running rich for a long time isn't harmless — the excess fuel washes the oil film off the cylinder walls (premature wear), dilutes the engine oil in the sump on a 4-stroke, fouls the spark plug, and can destroy the catalytic converter through overheating. Always start with the free, trivial causes — choke and filter — before considering a carburetion adjustment or an electronic diagnosis.