PRINCIPLES GASOLINE/DIESEL FUEL SYSTEMS - OD1620 - LESSON 1/TASK 2
(c) The needle valve is closed by the rising float as the fuel
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.