carburetor boiling over

I have a 65 mustang with a 68 302 w/autolite 2100. when I drive the car for about 15 min. and let it sit for about 4 min. the fuel in the float bowl starts coming out of the main fuel feed into the intake manifold. I have taken off the top of the carb and while this is happening, air bubbles are coming out of the check valve in the bottom of the float bowl(not the power valve). I have rebuilt the carb a few times, and the carb spacer is in place. my engine runs great,never stalls.My fuel tank,pick-up,fuel pump,filter,filter on carb inlet all new. If I tap on the carb with a screw driver while it is happening, it increases the amount of fuel dripping into the intake. It will continue until the bowl is nearly empty.