This is a weep hole for the throat; gas dripping into the thoat after shut-down (or when the carb floods) can drop out of it. If it leaks a tiny bit upon shut-down, that is normal for an updraft-carbureted engine. If it leaks a lot all the time, that is an overfilling bowl. Check out the info on the flooding page.

Usually there is a sintered metal “plug” pressed into the large diameter of the weep hole which would allow gas to drip out, but prevent most debris from being sucked in; it is missing on this carburetor for illustration. You can stuff a piece of felt in there and it will do the same thing, but you’ll have to figure out a way to keep it in place and change it realtively often.

It isn’t uncommon for a Marvel Schebler to have gas drip out of the air horn while sitting around. This is due to the gravity fed fuel design. To eliminate this problem, many installations have a shut off valve installed on the inlet fuel line. This allows the fuel to be shut off when not in use.