That vent circled in red was originally attached to a solenoid and then it ran down to the charcoal canister.
You can leave it open and try it, and then you can plug it and try it. On some vehicles I have noticed if I plug it, it makes it a little harder to start on a hot day, but all vehicles are a little different.
You can think of that port doing the same thing as those new aggravating fuel cans they have now. Those new cans do not let any gas fumes escape from the can until you mess with the buttons and are getting ready to pour the gasoline out of the gas can.
That port is part of the "evap" system and gas fumes from the carb are routed to the charcoal canister where they are stored there, along with your gas tank fumes which are sealed, and the gas tank has it's own breather line running up the frame to the charcoal canister.
The charcoal canister stores all these fumes and then when you start your engine and conditions are right, they had another line called the "purge" line which let the engine suck these stored fumes out of the canister and into the engine to be burned.