The Stovebolt Site Forums The Shop Area Engine Shop carburetor swap?

I recently purchased a 1972 gmc with a 350 in it. The truck is in great shape and runs fairly well. The truck has a four barrel holley racing carb in it and I would like to replace it with a stock two barrel carb. Do I need to replace the intake manifolds and does anyone know of a good two barrel that would give me better fuel economy on the low end. The truck is a short bed auto with 4wd and I am getting killed in gas. Through 5 tanks of gas I am averaging less than eight miles to the gallon The truck is used to cruise and I rarely drive the truck more than 40 miles per hour. Any ideas would be appreciated. Thanks in advance for your responses.

Chevy 350's in 4-wheel drive trucks are not noted for fuel economy, especially in the early 1070's.

As a general rule, maximum fuel economy will be realized by the use of a spread-bore style (Rochester Q-Jet or Carter TQ) 4-barrel. These have small primary venturi (for good low RPM air velocity) and "demand" secondary venturi which will provide sufficient air/fuel mix for power.

You did not specify the type of manifold, but a spread-bore carb should be used on a spread-bore manifold; just as a square-bore carb should be used on a square-bore manifold.

The square to spread adapters should be used only in an emergency, with much lower average engine efficiency.

To answer your question concerning the two-barrel, maximum two-barrel efficiency (lower than 4-barrel efficiency) would probably be obtained with a Rochester 2-barrel and an original Chevrolet 350 2-barrel manifold. We expect 15 to 20 percent better fuel economy from the 4-barrel under light load driving conditions. Obviously, the 4-barrel is going to use more fuel than the two-barrel if the driver fanticizes the "stop light grand prix".

Jon.