Balance Tube

Principles of Gasoline And Disesel Fuel Systems
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reaches the desired level in the float bowl.

(3) As the engine uses the gasoline from the float bowl, the level

will drop. This will cause the float to drop, which will open the needle

valve to let in more fuel.

(4) Venting (figure 13 on the following page). The pressure in the

float bowl must be regulated to assure the proper delivery of fuel and

purging of vapors. The following systems and devices are added to the float

circuit system to provide for these needs.

(a) Balance Tube. Due to the restrictions imposed by the air

filter and changing air velocities because of varying engine speeds, the air

pressure in the air horn is usually lower than atmospheric pressure. The

pressure in the float bowl must equal that of the air horn in order for the

carburetor to provide fuel delivery. A tube called a balance tube is run

between the air horn and the float bowl to accomplish this task.