[DanEC]

If you have a Holley or similar carb and look down the primary throats you should see a couple small retangular slots on the front just above the throttle plate shaft level. Those are the idle fuel slots and the idea is that when the throttle is resting closed on the idle adjustment screw - the thottle plate should be in about the center of the slot height. If your throttle plates are resting at the top of it, then that is why you aren't getting much adjustment out of your idle mixture screws.

There are a number of ways to deal with that (throttle setting is above fuel slots) and it could be that your timing is retarded at idle requiring that the idle setting is opened to much to compensate for it. But we don't know enough about your motor or how it's tuned to be sure. You may have a big cam, etc. But since you say you aren't getting much response to idle mixture adjustments on your vacuum gage, there is a fair chance this is part of the problem.

If you find this to be the case and you are otherwise pretty happy with how the engine is tuned (timing, idle speed) then removing the carb and slightly opening the secondary throttle stop a little so that the primary side can be closed a little, could help things out. A lot of people use to drill the primary throttle plates with small holes to pass air so the plate could be closed, but that shouldn't be necessary if the secondary side can be opened a little more.

I mention all of this as I have a fairly well built BB Corvette that used to diesel for this same reason. Once I got the carb all sorted out for the big cam it would shut off just fine.