[srmmmm]

I was fortunate to have three roommates at General Motors Institute that did their co-op at Rochester Products in the 70's. So I learned a lot from them on the Quadrajet design and tuning features, and had access to a good variety of jets and rods. One of them got me an 800cfm model for a Buick GS455 Stage-I that I modified the throttle arm on to fit my Firebird linkage. I ran the carb for many years on my 1970 350 without issue.
For the restoration of the Firebird, I had the carb refurbished and rejetted for the upgraded engine. Although a bit on the rich side, it ran very well on the dyno. Three years later (after covid interruptions), it's finally in the car and runs terrible. It starts fine and runs well as the choke pulls off on the fast idle cam, but refuses to idle when fully warmed up. I have to set the idle stop screw at the point where the engine is at 1500 rpm to run. It doesn't matter where the idle mixture screws are set. I can close them all the way or take them out all the way so the engine must be running on the main metering all the time.
I've even put the lightest power piston spring in from Cliff R to try and eliminate any issue there, but to no avail. Here's what is in the carb right now:

74 jets with 45B rods
CH Secondary rods on an "R" hanger
Air valve spring set at 3/4 turn tension
Power piston spring is Cliff's lightest

Engine runs extremely well from 2000 to 6000 rpm.

Engine:

1970 350 block bored .060 over
4.25" Eagle cast crank and 6.8" forged H-beam rods
Ross forged pistons, flat top, set at 0.0
6x-4 heads 9.3 compression ratio
Comp XE276HR 224/230 @ .050 110LSA
1.5 ratio rockers for .510 total lift
1-3/4" X 30" Hedman headers
MSD billet dist with 23 degrees mechanical set at 12 degrees initial

I'm thinking my next step to be a complete teardown of the throttle body but will gladly accept any suggestions.