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. See Vacuum Leaks

Idle too high. Should be 750 rpm or lower. See Carburetor_Adjustment#Idle_Speed_and_Mixture_Adjustment

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

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How do I stop the engine?

See the causes and solutions above. But for now, you can easily stop the engine:

set hand brake
push clutch pedal down and put in high gear
turn key OFF and at the same time let the clutch out slowly. This will stop the engine

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