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TM 9-8000
Figure 4-26. Float Circuit.
c. Venting (Fig. 4-27). The pressure in the float bowl
the fuel pump under pressure to the carburetor.  The
following events occur as the gasoline enters the
must be regulated to assure the proper delivery of fuel
carburetor through the fuel inlet:
and purging of vapors.  The following systems and
devices are added to the float circuit system to provide
(1) The gasoline begins to fill the float bowl.
for these needs.
(1) Balance Tube. Due to the restriction imposed
(2) The float rises with the level of the gasoline.
by the air filter and changing air velocities because of
(3) The needle valve is closed by the rising float
varying engine speeds, the air pressure in the air horn is
as the fuel reaches the desired level in the float bowl.
usually lower than atmospheric pressure. The pressure
in the float bowl must equal that of the air horn in order
(4) As the engine uses the gasoline from the
for the carburetor to provide fuel delivery. A tube called a
float bowl, the level will drop. This will cause the float to
balance tube is run between the air horn and the float
drop, which will open the needle valve to let in more fuel.
bowl to accomplish this task.
(2) Idle Vent. Because gasoline Is highly volatile,
Figure 4-27. Controlling Fuel Bowl Pressure


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