I have a 79' z28(fresh 350 .60 over) with a edelbrock 600 cfm carb. When i shut the car off it will sometimes clunk(diesel) for a sec. but just a few min ago it ran on for like 2-3 sec. What can i do to correct this? The car has the dual return springs so i don't think the carb is stuck open.

Thanks for any help,
Nick

 

Yeah, my 383 would do that too! I can't remember what I would do to prevent it.............I think you've gotta give it a couple quick revs before you shut it off? I think what causes it is a build-up of carbon in the cylinder, and when you shut off the engine (or try?! ) the carbon is still hot enough to ignite the small amount of gas that's drawn into the engine as it winds down. I could be wrong, but it's something to do with that, and high compression, or something?!

Actually, I do know what I did! I used to shut off the car in gear, then throw it into park once the engine quit!! But if you've got a 4-speed??????????? Stall it!

 

Yea i have a 4-speed i wouldn't think the motor has much carbon in it having only 1000 miles on it but i also read that too. The motor has a fairly high(10.3:1) comp. ratio whish probably adds to the problem

 

What octane of fuel are you running?I had a truck that would do the same thing on regular but if I ran premium it never dieseled.Something else to try is let the car idle for 30 seconds or so,shut the key off and just before it kills the engine turn it back on and then shut it off again.Those are some of the things I found that worked for me.

 

I run nothing but 92-93 octane

 

Could be timing related. What timing is your base timing?

 

Timing or too high of idle rpm can cause deiseling. Lean mix can as well but I bet its either timing or too high rpm on yours. What is base timing and what rpm is it when you shut it off?
CoryM

 

my brother in law's car dieseiled,so he asked me to take a look at it.when i popped the hood,i found the electric fan was wired to the +side of the distributer.the electric fan would keep spinning after the engition was shut off and create enough voltage for the distributer to spark for a few seconds after the car was shut off! that was funny!!1

 

I second this. Its exactly how I cured my '77.

 

My car idles at around 600-700 rpms so i don't think its that, so it's probably is the timing huh? I don't have a timing light so i had a shop set up the timing last fall after we rebiult the motor.(my dad wanted to have someone check it after we rebuilt it) My uncle has a timing light so i might borrow it from him and check it out

 

If your base timing is less than 12-14* at idle you shouldn't have a dieseling problem. However, many people have their distributor's vacuum advance hooked up to a full manifold vacuum port which causes it to kick in even at idle. Try to find a port on the carb that only supplies vacuum above idle speeds (a ported vacuum source) and use that. I've cured more than just one or two run-on problems with this simple change.