’71 Mark III Dieseling At Shut Off

Bill –
I recently purchased a Mark III, it is in excellent condition, very original and I’m having just one problem. Very shortly after purchasing the vehicle it started to run after shutoff very rough. It makes knocking sounds and shortly after finally shuts off. It does not do this every time. Sometimes upon start up I can faintly hear it as it starts for only literally less than a second. At shut off when said problem presents itself I have seen smoke leaving the air intake tube located by battery. I only use 93 octane gas. I have no previous knowledge of the cars service history. It has 66K miles. Very clean engine bay. What could cause this dieseling?? Otherwise the engine runs great with good power. Please help.
John
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John –
Hi John –
Congratulations on your 71 MK III purchase. They are great cars. The most common cause of dieseling at engine shutoff is too fast of an engine idle speed when the engine should be at its normal slow idle speed after the engine is at operating temperature. If this is the case with your engine and the problem started to appear on its own, the carburetor choke system and throttle mechanism should be inspected for a sticking condition. As you probably already know a cold engine will start and run initially on a fast idle and as it warms up the choke circuit will cause the fast idle cam to release the setting to the slower idle. If that or a sticking throttle shaft etc. is holding the engine at an abnormal higher idle after warm up the engine can “run on” or “diesel” after engine shut off. The above is the easiest and best way to first approach this problem. Knowing how the carburetor systems operate will be required and the complete carburetor operation is explained in the Shop Manual.
Sincerely,
Bill