Six Pack Center Carb

[Challenger6]

  Curious about something My Six Pack Center carb has a mechanical linkage that closes a vent on top of the center carb at the bowl on the front of the carb at around 20% throttle and further. At idle or less than 20% throttle this vent is open. The vent cap is a small rubber disc and the linkage runs towards the carb, turns towards the drivers side, then turns down the side of the carb and the mechanical linkage is captured behind a leg off the throttle linkage. Really like to know what this is supposed to do. I have heard that it was an attempt at early emissions control and have also been told that this center carb is very desireable because of this vent. It is the original carb that came on my '70 Challenger. Any ideas? 

[Chryco Psycho]

that is the 69-70 carb & instead of trapping the fumes it just opens the bowl to prevent the fuel boiling over into the intake& flooding the engine

[Moparal]

that is the 69-70 carb & instead of trapping the fumes it just opens the bowl to prevent the fuel boiling over into the intake& flooding the engine

and I believe it hooks to your fuel vent return line.    Ca emissions

[Chryco Psycho]

possibly but i doubt it , the cali carb has a hose nipple on the side of the bowl & is not open to the atmosphere , the way he described it it sounds like it just has the rubber seal on th etop of the bowl that is open to the atmosphere