ok i did a little refreshing of my AFB knowledge and i think i might know what your problem is, this from the edelbrock tuning guide at www.edelbrock.com
about the idle circuit
"...and the highly aerated mixture then
moves through the passages in the main-body to the location of the Transfer Slot (7) and Idle Screw Port
(8). The Transfer Slot (7) is a large air bleed when the throttle is closed, but as the throttle is opened the
slot is exposed to manifold vacuum and becomes a discharge port for Idle System fuel. The Idle Screw Port
is a variable discharge restriction that is adjusted by the engine tuner to achieve the desired A/F Ratio at
engine idle. "
if you look in your carb in the primary barrels youll see on the front side of the barrel two small holes, the one on the bottom is the idle screw port where the fuel at idle SHOULD only come out, but for example, on your motor, youve increased the idling rpm which theoretically will start drawing fuel out of the transfer slot which is the slot hole above the small pin sized hole in the barrel.
I would think the solution would be to drill two very small holes in the primary throttle blades so you dont have to have the throttle open so much. Ive seen holleys that had this done for similar reasons. Im no carter expert so if i were you id pick up a carter book (i think dave vizzard wrote one??) and im sure they would have this info in there
im assuming you are just running a rich idle