My car did the same thing after rebuilding the carb....found this informative & probably appies to most carburetors.
"Idle transfer slots
Pay close attention to the idle transfer slots(rectangular)in the front of the primary bores of the base plate. When the carb is correct in cfm's and jetting, the slots will only show a square when the butterflies are in the normal idle postition. If the slot is a rather large exposed area, the carb will need the inital timing bumped up which will increase the idle speed and will allow the idle speed screw to be backed down, thus exposing less of the transfer slot. If the slot is less than a square, back off the inital timing and crank up the idle speed."
