I had this same issue a while back with a Holley carb I was running, and turned out it was a stuck needle and seat, flooding over into the carb out of the vent tube...I tapped it with a small hammer to get it home, then changed the needle and seat on the primary side.
1967 Firebird 400/4spd all numbers matching 1971 Camaro Pro ET Drag Car 1970 Camaro real Z-28 (needs everything) 1963 Impala 409 dual quad 4 spd 2003 Victory Vegas 2009 Pontiac G8 GT (My wife says I have a problem.....)