Large rpm drop

Charlesvolare said
fuel pressure at 6psi,
float level checked,

Pressure and float level are good to know.
These should result in the proper fuel level.
The way to check that is at the site plugs. Depending on the carb, fuel should either be just below the plug, or in the mid area of the glass.

Charlesvolare said
Transfer slot at 0.040" secondaries barely cracked open timing at 30*
idle mixture screws 1 turn out from seated,
14-15 inhg on vacuum gauge plugged into full manifold vacuum.
~600 rpm in neutral, ~450 in gear, nothing higher.
Black, rich smoke in exhaust, enough to fog surrounding neighbors.

And previous it was:
20* @ 1200 rpm
T-slots @ something much more than .040"
~600 in gear

14" in at 600 rpm is pretty strong. That should be enough to pull fuel through the idle ports.
I'm not thrilled about 30* BTC to get that though.
I was expecting 9 to 12" Hg with somehting close to the 20* BTC at 800-1000 rpm.

I'd try some air first. Crack the secondaries more (2 corner idle)
I should have asked, is there a PVC valve?