As was said its a bleed off valve for the Idle Enrichment System. It uses either a coolant vacuum switch or a solenoid to control the vacuum to the valve in the carb that controls an idle air bleed so at times it can richen the idle circuit some more when the choke is on. When the vacuum valve or solenoid closes it could trap some vacuum in that system and not let the idle enrichment valve turn all the way off. So its basically a very small vacuum leak when it has vacuum applied to it. But its so small that it does not bleed off enough vacuum to stop the enrichment valve from working. And it will bleed off any trapped vacuum when the vacuum valve or solenoid turns it off. It has that small brass type filter to keep dirt out because the hole can plug up real easy since its so small. I don't think I have seen one for about 30 years. Ron