Pre-ignition means that the mixture is set on fire before the spark occurs, being ignited by a red hot spot in the combustion chamber such as an incandescent particle of carbon; a thin piece of protruding metal; an overheated spark plug, or a bright red hot exhaust valve. The result is reduction of power and overheating accompanied by pinging. The bright red hot exhaust valve may be due to a leak, to lack of tappet clearance, to valve sticking, or a weak or broken spring. Pre-ignition may not be noticed if not severe. Severe pre-ignition results in severe pinging. The most common cause of pre-ignition is a badly overheated engine.
When the engine won't stop when the ignition is shut off, the cause is often due to red hot carbon particles resting on heavy carbon deposit in a very hot engine.
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