If you could connect everything that you are sure of, and then take a pic of the carb and post it, you might jog someone's memory and get the answer you're looking for. Its difficult to guess where that port connects to. There are several different versions of these carbs and the vacuum systems are complex. You do know that if you plug the port the engine stalls. This tells us that the port connects to something that allows a continuous air flow through, so it would not connect to a diaphragm like the EGR for example.