Stumble after shutoff

[georgeramos]

My car had been occasionally stumbling for a couple of seconds after shot off. It doesn’t happen every time but what might be the cause of such a symptom?

Thanks all!

[18Fiatsandcounting]

The most likely cause is that something in the combustion chamber(s) is still hot enough to continue to ignite the fuel even after the ignition (spark plugs) is turned off. Check the heat range of your spark plugs to make sure they're not too hot for your engine. If the engine only exhibits this behavior when really hot, this is likely the cause.

Depending on what carburetor you have, there might also be an idle shut-off valve on the carb that isn't shutting off.

For the short term, you can also kill the engine by letting the clutch out just slightly with your foot on the brake. Not ideal, I know, but it's one way to get the engine to stop after you've turned it off.

-Bryan

[SteinOnkel]

In addition to what Bryan said, this may also be caused by:

- an overly rich mixture (especially in combination with an idle speed screw that is technically too high, but needs to be that high for the car to idle at all, because it's so rich)
- the alternator backfeeding the electric system, keeping the ignition coil firing even though the key is in the off position. This can be remedied by placing a diode into the exciter wire for the alternator

[georgeramos]

I did fiddle with the carb recently and it was idling higher than usual afterwards. I also just swapped the air cleaner element which was pretty old. I will re adjust the carb to see if it helps thanks