Okay, so when I turn the key off I'm getting a knocking/shaking and the engine sounds like it's trying to keep running.Happens maybe once or twice out of ten. From what I've been able to find on here I think the term is "dieseling" but I'm not sure. It sounds terrible and I'm sure its bad, can anyone tell me how to fix this? I have a 76 F250, just put a rebuilt 390 in it, about 1000 miles on it so far. New Holley 2 barrel 500 CFM carb. I'm not mechanically inclined but trying to learn so please speak noob language. Your help is appreciated, thank you.

 

Yes, this is called dieseling. It's caused by a combination of heightened combustion chamber temperatures and a path for air/fuel to enter the intake stream after shut-off. I'm going to guess that this engine used to have EGR, and that was left off the rebuild. That's the most common reason for heightened combustion chamber temperatures. Advanced ignition timing and lean running conditions cause the same thing. High idle speed only worsens the problem; some engines employ an idle stop solenoid for this reason.

 

So what do I need to do to fix it? Or should i just take it to a shop?

 

What rpm are you running at idle? Can we assume all the tune up parts are new also?

 

Not to be a smart aleck, but the answer is, "everything in my first post."

1) Investigate if the motor has provisions for, or originally came with EGR
2) Check timing
3) Check idle speed

Whether or not you take it to a shop for that is your call. If you're interested in doing this yourself, I highly recommend picking up a Chilton's or set of factory shop manuals to get yourself started.

 

Thank you.

 

As FMC400 said, get a Chiltons and Haynes books.

In the "Troubleshooting" section, there is an entry for "dieseling". It gives the reasons and points you to the chapters that remedy it. Even if you dont tackle it yourself, it would be a good for you to start learning a bit.