Engine run on is caused from an engine running extremely hot so if your timing is advanced to much it could cause it, and from foreign material in the cylinder head (carbon) being hot enough to cause ignition.
From Wiki:
This condition can occur for a multitude of reasons:
Built-up carbon in the ignition chamber can glow red after the engine is off, providing a mechanism for sparking unburnt fuel. Such a thing can happen when the engine runs very rich, depositing unspent fuel and particles on the pistons and valves. Similarly, non-smooth metal regions within the piston chamber can cause this same problem, since they can glow red. It has also been suggested that an improperly rated spark plug can retain heat and cause the same problem.
A carburetor that does not close entirely can contribute to running once the engine is off, since the extra fuel and oxygen mixture can combust easily in the warm piston chamber. Similarly, hot vaporized oil gases from the engine crankcase can provide ample fuel for dieseling.
Incorrect timing.
An engine that runs too hot or too lean may produce an environment conducive to allowing unspent fuel to combust.
An idle speed that is too fast can leave the engine with too much angular momentum upon shutdown, raising the chances that the engine can turnover and combust more fuel and lock itself into a cycle of continuous running.
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Today's Featured Article - The Nuts and Bolts of Fasteners - Part 2 - by Curtis Von Fange. In our previous article we discussed capscrews, bolts, and nuts along with their relative hardness and thread sizes. In this segment we will finish up on our fasteners and then work with ways to keep them from loosening up in the field. Capscrews, bolts and nuts are not the only means of holding two parts together. When dealing with thinner metals like sheet tin, a long bolt and
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