From Rochester manual:
"An important design feature used in the Rochester 2-bore carburetor is throttled body venting. Its purpose is to give a quicker hot engine starting after the engine has been shut down for a short period. During extreme hot engine operation, the fuel in the carburetor tends to boil and vaporize due to engine
heat. Some of the fuel vapor tends to reach the carburetor bores and condense on the throttle valves and seep into the engine manifold; by venting the area just above the throttle valves, hot engine starting time can be reduced to a minimum, on applications where the carburetor is exposed to extreme engine heat. There are two methods used in venting the throttle bore area.
1. A special throttle body to bowl gasket is used. See Figure 5A. This gasket has cut-out areas that vent fuel vapors from the carburetor bores just above the throttle valves.
2. The other type of venting is accomplished by drilled holes through the throttle body casting just above the throttle valves. See Figure 5B. They serve the same purpose as the vented gasket.
The location of the vent holes is such that they will not disrupt engine idle or off-idle operation. They are
located above the throttle valves on the side opposite the mixture screws, in an area where the transfer
from idle to the main metering will not be affected."
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