Why does my engine still try to run after I shut it off?

Dieseling or engine run-on is a condition that can occur in gasoline powered engines, whereby the engine keeps running for a short period after being turned off, due to fuel igniting without a spark.

Causes of "running on" (continuing to run when ignition is turned off) are most commonly these:

Air leak -- leak at manifold gasket, carburetor gasket, or any of the assorted vacuum diaphragms such as the distributor, air cleaner flap motor, or carburetor choke-pulloff.

Idle too high. Should be 750 rpm or lower.

Open Chamber type cylinder heads. While these reduce emissions, they are more susceptible to run-on. Datsun fitted carburetors with Anti-diesel valve to these engines.

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