The most common cause is the engine idleing to fast. Timing being a little to far advanced can contribute to the problem also. A fuel injected engine, when shut off, gets both, spark and fuel shut off, a carbureted engine only gets the spark shut off. If there is something hot enough in the combustion chamber to ignite fuel, such as a hot spark plug tip, or glowing carbon chunk, the engine continues to draw in fuel, causing the run on condition. Check your idle speed and timing first. If that doesn't do it you can dump some Sea Foam down the carb, with the hot engine at a fast idle. Be careful you don't want to hydro-lock the engine, just dribble it down the throat of the carb, shut it off then let it sit for a half hour or so. This should help de-carbon the tops of the pistons
