74 Duster with 408 Stroker With Dieseling/Idle Issue.

Remove your carb and turn it upside down. There is a vertical slot in each base or throttle plate, the throttle blades close by it. If your idle screw is turned in too far, this slot appears to be rectangular instead of square. I'm sorry, I do not have a spare Holley carb to take a picture.
Maybe someone else can take a picture, or you can Google it. Idle transfer slot, it allows fuel to enter at idle. Getting that slot close to square stopped my run on.