Ohboy... where to start?
Congratulations on choosing to use methods that most have forgotten about. And for using your senses. You'd be amazed at how many kids these days are "Toonerz" because they have a laptop and an exhaust oxygen sensor. Few develop a sense of what's happening like the folks that spent years actually wrenching on engines.
The vacuum gauge is a way to determine how much the engine wants to accelerate compared to how much the throttle is holding it back. In the old days guys would set the timing for maximum vacuum at idle. The fuel mixture screws would be adjusted in and out while watching the vacuum gauge and like the timing, if the gauge dropped when the screw was turned that meant you were going the wrong way. Eventually both the timing and mixture screws would be at the point where engine vacuum was strongest and the idle speed would be given a final set.
Engines today are not always set for maximum vacuuum as that can increase tailpipe emissions. And cams are not always ground to deliver smooth idle. So how about some details? Without knowing heads, cam, and compression ratio, it's going to be tougher to cover everything.
As far as stated goals, what I'm seeing is that you want to stop the "dieseling" that occurs after the engine is shut off. The answer to this is simple: Decrease fuel and air allowed into the engine to the point that that compression ignition does not occur.
Generally speaking you are going to want to balance reducing the throttle angle with setting timing. If the cam grind creates a rough idle when the throttle is closed then you are likely to end up with a rough idle. If the cam grind has a large amount of overlap and encourages exhaust dilution at idle then you are likely to smell a rich exhaust when you are done.
The engine is running at 1000 rpm, the idle mixture screws are all the way in, spark advance is probably advanced over original, and the exhaust smells rich. If your carburetor is not leaking then there's a chance that your engine's combination of cam/heads/compression ratio are going to smell rich at idle.
The idle screws should not be closed. In order to get enough fuel for the engine to operate they should be open about 1 to 1 and 1/2 turns. Start there.
Check to see if you have ported or manifold vacuum at the distributor. Ported vacuum is a source where no vacuum is present when the throttle plates are it idle but is present when the throttle plates are opened. If you have vacuum at idle the result can be extra advance at idle which makes it harder to idle the engine down. It can also cause vacuum advance to begin operating during cranking which can be a real challenge on hot days. I tend to prefer ported vacuum on mild engines. If ported vacuum is not available be prepared to decrease timing slightly or disconnect and plug the vacuum line during testing.
After opening mixture screws and getting the vacuum situated, check to see if the throttle angle needs to be decreased. If decreasing timing reduced engine speed that might be enough to cure the run-on.
Of course this might be all wrong if you're running a fairly stock '80s engine. Those were often set up with little advance timing, throttle plate open at idle, and relatively low vacuum.
Yep, details might help.
