Fresh 496 Running Rich. Ideas?

[gizmoduck81]

Hi everyone. Hoping you can help. 1963 GMC with a freshly built 496 (Mark IV) that is running really rich at idle and I’m out of ideas. Plugs are gapped at .045. It is hard to start and won’t start unless the choke is fully open. Even then, it takes several cranks and doesn’t run smoothly until warm. The idle mixture screws are out only 5/8 of a turn and that keeps the AFR at idle around 13 (with 1.5 turns out it’s running at 10 AFR if it will even start). The engine does die if I turn them all the way in. I have tried initial timing from 10 up to 22. 22 is where it is now but that seems excessively high to me. It was at 10 initial/34 total at the dyno and I originally changed nothing when installing into the truck. Timing was read with vacuum advance properly plugged. On my second set of plugs and wires just to rule those out. HEI is properly wired and receiving 12V. Checked for vacuum leaks and found nothing. Any ideas for what else I can check? Is this just the nature of the beast? Defective carb? I’ve searched around this site and the internet in general and I feel like I’ve tried everything.

Heads: AFR 3610-1 265cc heads
Cam: Comp XE274H, Int/Ext @ .050 230/236, Lobe separation 110
Compression: 10.06:1 static, 8.21:1 dynamic
Gas: Pump 92
Fuel pump: Mechanical @ 5.5 – 6 psi
Plugs: Autolite 3924 gapped at .045
HEI: MSD Pro Billet
Carb: New Holley 4150 Street Avenger 870cfm, 6.5 power valve, 79 primary, 83 secondary
Vacuum at idle: ~12-13

Let me know if I’ve left any important info out. I can post scans of engine build/dyno runs if that helps.

Thanks!

Ryan