An unvented fuel tank can push gas past the needle/seat when the temperature goes up. Plugged bleeds can lead to a siphon effect in the carburetor but would affect how it runs. Is it rich? The AFB/AVS carbs aren't supposed to do this (leak below float level).
THIS; Had an issue w/a billet 850ish carb I built for Shane Studly and his stocker Mopar wagon. Ran great till he topped off the tank then the pressure in the tank literally crushed the ft. brass float(which I no longer use in builds) sticking the needle n seat flooding the car to the point of filling the crankcase w/gallons of gas. Solution, He added a vent to the filler neck and problem solved.....Especially important w/an elec. fuel pump....
