Thanks Mattax! Everything you wrote is very helpful and crystal clear.
My carb is an Edelbrock, not a vintage Carter, but I don't believe the carb has been modified. True, it's non-stock.
I should have mentioned...since my first post I have switched from manifold to ported for the vacuum advance.
So I have 15-16 degrees BTDC initial. The distributor is a "4-seconds flat" unit that he specifically did the mechanical and vacuum advance for a 440 with iron heads, stock cam, 4-speed and 3.23 gears. If I plug in the vacuum advance to manifold vacuum it jumps from 15-16 to 30 BTDC.
I put it back to ported vacuum.
According to all the Edelbrock videos I have seen, a good starting point for mixture screws is 1 1/2 turns, not a 1/4 turn. That is what is bothering me. And yes, if the timing is not right and the throttle position is not right then the idle mixture is moot.