Throttle Body Venting
Some Rochester 2 Jet carburetors include what is called throttle body venting. Gas now has a lower boiling point than it used to so this can be a very useful feature these days.
The idea of this article isn’t so that you can go out and add venting to your carburetor, but to help you understand this feature, so if you are having percolation problems, it could be that the throttle body vent is plugged off somehow, probably by the gasket.
Percolation – gas boils from hot conditions, turning it to vapor, or boiling over into the carburetor bore.
There are 2 types of venting. A vented throttle body gasket as shown in figure A is sometimes used. This gasket has cut out areas which vent fuel vapors from the carburetor bores just above the throttle valves.
The 2nd type has vent holes drilled into throttle body, just above the throttle valves and serve the same purpose as the gasket vented slots. The location of these vent holes are such that they don’t disrupt idle, or non idle operation. The holes are located above the vent valves on the opposite side of the idle mixture screws in an area where the transfer from idle to main metering will not be affected.
When rebuilding your Rochester 2 Jet carburetor be sure to check these holes. They need to be open and clear.