Car doesn't want to stop running?

[krandall]

Hey guys,

question for you motor heads..? I've noticed on my Coronet that every once in a while when I go to shut it off it will keep trying to run. It'll fire/chug a little bit and eventually stop. I'm fairly green horn to troubleshooting it so just looking for any tips to try or what I should look into first.
Thoughts?
thanks for your help!
-randy

 

[super-bee_ski]

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

It is simply called dieseling because it is the same way diesel engines operate: by firing without a spark.
Remember: the ignition source in a diesel is the compression of the fuel in the cylinder, rather than a spark as in gasoline engines.
The dieseling phenomenon occurs not because the compression ratio is sufficient to cause auto-ignition of the fuel, but a hot spot inside the cylinder starts combustion.


Some causes:
1. A carburetor that does not completely close 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.
2. Incorrect timing.
3. An engine that runs too hot or too lean may produce an environment conducive to allowing unspent fuel to burn.
4. 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 turn over and burn more fuel and lock itself into a cycle of continuous running.

 

[krandall]

Awesome. This gives me some places to start. Thanks much Super-bee!!!
-randy

 

[Dako]

I would go in this order (easy to more involved)

If you don't open it up from time to time, take it out on the open road and 'blow it out'. That old adage of 'burning off the carbon' can have a bit of merit (although I don't know many enthusiasts that have to worry about carbon build-up Ha-ha!)

Check your idle rpm.

Check your fuel/ air mixture screws. With the car running, turn one side in until seated (not tight!...you can damage the seats), then back it out 1 1/4 turns. Do the same to the other mixture screw. This is a good starting point. With a vacuum gauge you can dial the mixture right in.

Check your timing. Incorrect timing can cause this.

Check your spark plugs to see that they are a nice gray/ tan color and that they are correct for what is called for in your particular engine.

Finally the carb...i.e. carburetor not closing completely.

 

[super-bee_ski]

Thanks Dako....
Good point....

 

[coloradodave]

super-bee_ski: I'm impressed. That was better than Wikipedia....and it was correct to boot. Great answer.