Reply to "Idle Vaccuum"

No it is because of the total advance you have dialed in and you probably have a vacuum advance distributor?

Look for a vacuum advance limit switch. It is used in line on many Ford products since about 1975 or so.

It is a simple device that you just install in line in the vacuum hose to the distributor.

This will reduce the strength of the signal to the vacuum advance in the distributor and should limit or eliminate the dieseling.

Other than that you would need an electronic ignition system that retards the ignition when you shut the ignition off. I don't know of any that exist right now for that though?

Fuel injection will also stop this from happening by shutting off the fuel supply to the engine.

You are caught in the middle between the old technology and the new ones that fixed these issues as a result of dealing with "emissions requirements".

17 degrees is also a little on the high side. You may need to slowly dial it down until you stop the dieseling. I would guess 15 degrees will do it.

Another consideration if this doesn't work is that it many not be the total advance, it may be the rate of advance in the distributor.

In my experience you need to use the heavier mechanical advance springs so that total advance is not in until a minimum of 4500 rpm if not 5000rpm.

The light springs cut that in half, or advance twice as fast. They would be all in by 2,500rpm.

Todays pump gas will not permit that. Faster advance creates higher cylinder pressures just like a higher static compression ratio would and the gas can not take the added pressure without detonating, aka, dieseling.

High octane racing gas such as Sunoco 106 will have greater resistance to being compressed into firing without a spark, again, aka, diesleing.