Vacuum in the Rochester 4G

This shows the air movement in a particular 4jet style base. The vacuum draws air for the automatic choke from a tube running through the heat crossover passage, the heated air passes through the choke housing, moves into the carburetor flange, goes around the primary bores and then is drawn into the manifold.
This system helps prevent icing in the carburetor by heating the throttle valve and idle port areas as the air passes through.

Some carburetors use a heating grid between the manifold and the carburetor which would look something like this:

Once the engine heats up to temp the heating grid shuts off.

Updated on 04/28/2022

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