[bobbader]

This IS the idle port. Some B & C carbs have a hole in the front center of the throat like an A carb. Others have this brass "plug" with a slot in it. The slot goes half way through the brass plug where there is a hole that matches the gas port that comes up through the idle passageway. If the carb has been bead blasted, it has possibly clogged the slot and/or the hole passage or it is otherwise clogged from years of rust and debris. There is a passage plug
in the top flange that, like those pesky plugs on an A carb, when drilled out, will give access to clean the passage hole. If, however, the slot is clogged, there may be no way to clean it without removing the slotted plug. It is accessed on the engine side of the front of the carb throat and can be removed by carefully pushing through to the inside of the throat with a soft punch.

Good luck. PM me if you need further assistance.

Bob Bader