For my 1968 mercury cougar it has a 390 in it and after you drive it or let it run for a while it wants to keep running after you turn it off. it has a pertonix in it and a carb that has an electric choke on it. I was wondering what i can do to stop the run-off/dieseling
Start with buying the highest octane gasoline you can get. Run a tank and report back. Check the timing mark too and let us know where it is at.
I run 91 in it and that’s the highest we have here
Try turning the idle down.
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Lower idle speed. Turn engine off while in drive or reverse (if auto trans).
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It be on the carburetor right
You could look into fitting anti dieseling solenoid. What it does is set the idle RPM lower when the key is turned off preventing fuel getting to the engine. They were fitted to many vehicles when emission requirement became more stringent. Most carburettor manufacturers have some version of their own.
Also check to see if vacuum advance is connected to ported or manifold vacuum. I read you have new carburettor, hence it maybe connected up wrong. If it is connected to ported, it could be running too much retarded at idle causing engine to overheat, hence the over run.
(It hard to say with ported or not without actually being there and I am not familiar with the 1968 390?)
Also check to make sure the choke is coming off when engine warms up.
Peter 