Difficulties finding carburetor fuel inlet

[Webble]

Hello,
I recently bought a 1977 cb750 k7, however, the fuel lines weren't connected. It looks like the carburetors have two inlets on top, but I can't find which one connects to the fuel tank or if both have to be connected. Could anyone point out what each inlet is used for?

 

[dirtdigger]

#2 is the fuel inlet. #1 is the vent for the float bowls

 

[madmtnmotors]

dirtdigger is correct. The vent (#1) would have originally had a short (6" or 8") hose routed back to a clip on top of the air cleaner, pointed toward the rear, open to the atmosphere.