Rochester 4 Jet Float Bowl Vent
Some of the 4 jets have a float bowl vent on top of the carburetor. Not all have this particular vent.
The vent is operated by the accelerator pump lever and is used to vent the float bowl so that the bowl doesn't bleed gas.
A - Rubber vent valve, which opens when the accelerator is pushed down.
B - Accelerator pump Arm, which controls the pump and the vent.
C - Accelerator pump.