The internal combustion engine in a lawnmower ignites the fuel in the cylinder at incredibly high temperatures. However, this heat shouldn't ignite anything else in the engine. A smoking engine is a precursor to engine fire; if any type or colour of smoke starts emanating from your engine, stop the engine immediately and locate the source of the smoke.
Fuel Burning Too Hot
If the fuel inside the cylinder is burning too hot, it can cause the engine to start smoking. Fuel will burn too hot when it starts going bad. Fuel ages quickly; and after a little less than 30 days, the gas will be bad. Bad gas will leave behind gummy deposits of fuel across the engine, and this may reignite in the cylinder or carburettor, causing smoke to pour from the engine. Check the condition and age of your fuel. If any old or bad gas is sitting in your tank, dump it out into an approved fuel container. Old or bad gas may require a cleaning of the entire fuel system.
Another common source of engine smoke is from the muffler when trapped carbon deposits reignite and start burning. The spark arrester screen stops all ignited embers leaving the engine; however, this screen will eventually clog with black carbon deposits. When this black carbon collects, it can reignite inside the muffler. Clean the spark arrester screen with a wire brush and soapy water. Scrub out the muffler and exhaust port if necessary. Replace the muffler if it can't be cleaned.
Oil originates from the tank, where it gets pumped throughout the crankcase and cylinder. If the oil can't flow unobstructed through the system, it may create a jam, which can eventually burst a hole in the oiling system. When oil starts leaking from the system, it can ignite and start smoking. These oil leaks occur frequently around the base of the engine, near the crankcase, and at the base of the oil tank, near the oil fill O-ring. Inspect the engine and oil tank thoroughly for any signs of leakage.
Concurrently, the fuel system can also experience the same ruptures, causing leaks of not oil, but fuel. Fuel will also ignite outside of the cylinder if the fuel leaks into the intake manifold. Bad fuel deposits also dirty carburettors, which can create a backup and engine smoke. Carburettor smoke will often emanate from near the fuel tank and air filter and will be whiter in colour. Disassemble, clean and readjust the carburettor.