The Rochester B has a unique idle system called 'snatch-idle'. "The idle tube is located across the nozzle from the main fuel system. Idle fuel is 'snatched' across the air gap into the idle down passage. One side benefit of this system is that the main nozzle passage is always full of fuel and ready to provide it on demand." From Roe and Fisher's 'Rochester Carburetors'
However, if what you are seeing is fuel coming out of the bowl vent, I don't think its the idle system. As suggested above, your float level may be off. (Just rebuilt my Rochester B and mine was off by 1/4 in drop and 1/8 with the needle valve closed.
You might also want to check the fuel pressure. I think the B is supposed to be supplied with 2-6 psi. That new fuel pump might be pushing too hard. If so, add a regulator.
