An important design feature in pre-1971 Rochester 2-barrel carburetors was throttle-body venting. It gave quicker hot-engine starts after short shut-downs.
During extreme hot-engine operation, fuel in the carburetor tends to boil and vaporize. Some of the fuel vapor reaches the carburetor bores, condenses on the throttle valves and seeps into the engine manifold. By venting the area just above the throttle valves, hot-engine-starting time can be reduced to a minimum.
This was done by one of two methods:
A special throttle-body to bowl gasket that has cut-out areas which vent fuel vapors from the carburetor bores just above the throttle valves to atmosphere. The vapors are emitted to the under-hood area.
Holes through the throttle-body casting just above the throttle valves serve the same purpose as the vented gasket. The vent holes are located so they will not disrupt idle or off-idle operation. They are above the throttle valves on the side opposite the mixture screws, in an area where the transfer from idle to main metering will not be affected.
Starting in the early 70’s, these vents, and cut-out gaskets, were eliminated to reduce evaporative losses.
Examples of vented throttle body gaskets:
