How a vacuum fuel system works

April 1, 2008 | By Staff

Recently someone asked about the vacuum fuel system and wanted to know how it worked. I hate to say this, but you did not answer his question. From the early teens until AC invented the lever diaphragm fuel pump (I think in 1932), everyone used the Stewart Warner vacuum tank fuel system.

Basically, there was a small tank that mounted on the cowl. This tank had a small tank inside of it.

Vacuum from the intake manifold caused a partial vacuum inside the inner tank. Atmospheric pressure then pushed gas from the gas tank to the inner tank until atmospheric pressure was equal. Inside the inner tank was a float. When the inner tank was filled, the float shut off valves to the intake manifold and restored the tank to atmospheric pres- sure. The inner tank had a flapper valve in the bottom; once atmospheric pres- sure was restored the flap opened and it drained into the outer tank.

Since the whole thing was above the carburetor, gravity then brought the gas to the carb.

Once the inner tank had emptied, valves closed it to the atmosphere, the valve opened to manifold vacuum, and the whole thing started again.

Unlike a fuel pump, it took no horse- power from the engine to operate the vacuum system.