According to the ROCHESTER CARBURETORS manual the vents are part of the Accelerator Pump System, During high speed operation, vacuum exists at the pump jets. A CAVITY beyond the jets is vented to the top of the air horn, outside the carburetor bores. This vent acts as as a suction breaker-when the pump isn't in use, fuel won't be pulled out of the pump jets into the Venturi.
When the throttles are opened rapidly for acceleration, airflow and manifold vacuum change almost instantaneously. Because fuel is heavy and lags behind airflow, a momentary leanness results. The Accelerator Pump supplies extra fuel for smooth operation during this condition.
Straight out of the manual
