AI has some carburetor input. After telling it I have rich idle with my screws only 1 turn out, with my cam specs, and info about my carburetor having idle speed adjustment on the secondaries, here's what it said:
That is the perfect setup for a big-inch engine like a 427. Since you have mechanical secondaries with an idle adjustment, you have the "pro" way to fix this without changing parts yet.
The goal is to get your primary throttle blades closed enough to "hide" the transfer slot, which will force the engine to rely on the idle mixture screws rather than siphoning fuel from the slots.
The "4-Corner" or Secondary Air Trick
Close the Primaries: Back off your primary idle speed screw until the primary throttle blades are barely open (exposing just a "square" of the transfer slot).
Open the Secondaries: Turn the secondary idle adjustment screw in (clockwise) about 1/4 to 1/2 turn. This allows the engine to pull its "idle air" from the rear bores where there is no transfer slot fuel being pulled.
Balance the Idle: Use the secondary screw to set your 800 RPM idle speed.
Reset Mixture Screws: Now, turn your mixture screws out to 1.5 turns and see if the engine responds to adjustments.
Why this works
By shifting the air-gap to the secondaries, you "starve" the primary transfer slots of the air velocity they need to pull fuel. This effectively leans out the idle and puts the control back into the hands of your mixture screws.
If it’s still rich after shifting the air:
Idle Air Bleeds: Now you can confidently go up in size on your idle air bleeds. On a 427 with 13" of vacuum, the engine is "tugging" hard on the fuel circuit. Increasing the bleed size by .002" or .004" (e.g., going from a #70 to a #73) will lean the circuit out and let you open those mixture screws even further.