Sometimes that problem is caused by the throttle plate not being exactly centered in its bore. The fix is to loosen (NOT REMOVE) the two screws that retain the plate to the throttle shaft, back the curb idle screw off until it's not touching anything, and gently tap on the plate to center it, then retighten the screws. Those screws are peened on the end opposite the head, so they won't get sucked into the engine if one gets loose. Never remove those screws without grinding the peened-over part off or you'll ruin the threads in the shaft.
Jerry
