[joe_padavano]

As Dan noted, that's a bowl vent from the bad old days before evaporative emissions equipment. The purpose is to let fuel vapors escape if the fuel in the float bowl started to boil due to heat soak from a hot engine, otherwise the increase in pressure would force the fuel into the engine and lead to flooding on a hot start. That's why the vent opens when the throttle is closed.

There's no adjustment for this, it's simply a hole in the top of the float bowl. If the float is set too high, you will get leakage, but it sounds like you've taken care of that. The problem may just be that today's gasoline is not designed for use in older cars like this. The gas is probably a little more volatile and as a result, generates vapors in the float bowl more readily. Any car built after 1970 has an evaporative cannister that collects these vapors, so you don't see the leakage. Is this leak a running dribble, or just wetness that accumulates on the carb? If the latter, there may not be anything you can do about it except be sure the float is set to the low end of the tolerance band. Also, did you replace the float when you rebuilt the carb? Additives in today's gasoline attack older floats and cause them to be heavier than spec, effectively making the float setting higher than spec.