I have just spent some time with my carburetor ensuring that the throttle plates seat fully beyond the 2 idle transition holes and the vacuum advance pickup hole.
After messing with it, the very best running conditions I could get at 575 rpm in park were as follows:
- dist. at indicated 10* initial advance
- Idle speed screw 5/3 turns in from just touching tab on throttle shaft, throttle plates lightly seated
- Mixture screws each 5/8 turn out from lightly seated.
This setup yields a steady 575 rpm and a touch over 21"hg vacuum. Yet 5/3 turns has the throttle plates far enough open that the lowermost idle transition hole is fully exposed to manifold vacuum, as is the bottom edge of the advance port. Turning the mixture screws in or out even just 1/4 turn from the best position results in an obvious decline in idle quality.
I checked the power valve chamber for gas 3 times. The first time, even with a brand new valve and gasket, there was some fuel in it. I inspected the gasket, the valve, and the gasket seat and found no flaws, so I put it back together and over-tightened the power valve, which caused the gasket to shear halfway through its thickness. I installed it anyway and checked it twice. Both times the gasket was visibly wet but there was no gas collected in the cover as before.
Do you agree that 575 rpm should occur with less throttle and more mixture screw, indicating a problem?
As ever, thanks for patience with my long posts 
and for your wisdom.
1954 Crestline Victoria 312 4-bbl, 3-speed overdrive