Idle screw and backfire

[Larry Seemann]

KP, pulling the choke enriches the mixture. Therefor what you're describing indicates a lean mixture situation which is usually caused by a vacuum leak. A vacuum leak can be in the wiper circuit (if the car has a vacuum wiper), or leak where the manifold bolts to the block, or where the carb bolts to the manifold. Do you perchance have a Tillotson carb? They are famous for warping at the flange that bolts to the manifold and sucking in air there.