Engine builders often spend many hours removing sharp edges from engine components.
This is done not only to reduce stress risers and prevent parts breakage, but also to prevent hot spots that can cause pre-ignition. By removing the sharp edges with sand paper or other methods, the builder can continue assembling the engine with less fear of problems later on.
This often causes the engine to break pistons or bend connecting rods as the other cylinders attempt to fire in the correct timing sequence. The best-case scenario will result in engine knocking or pinging.
Pre-ignition as well as engine knock both drastically increase the temperature within the combustion chamber. This virtually ensures that the occurrence of one of these conditions will bring on the other. Pre-ignition results in poor engine performance and is often accompanied by a rough running engine.
To prevent this from happening, some engines have a "fuel cutoff solenoid" on the carburetor to stop the flow of fuel to the engine once the ignition is turned off. Others use an "idle stop solenoid" that closes the throttle completely to shut of the engine's air supply. If either of these devices is misadjusted or inoperative, run-on can be a problem. Engines with electronic fuel injection don't have this problem because the injectors stop spraying fuel as soon as the ignition is turned off.