Lesson 1/Learning Event 2
When fuel enters faster than the engine can use it, the bowl fills up. This causes the float to rise, and it
pushes the needle valve into its seat, shutting off incoming fuel. As the engine uses fuel, the fuel level in
the bowl drops and the float lowers. The needle valve opens, and more fuel is allowed to enter.
In actual operation, the needle valve is very seldom closed. Instead, it stays open just enough so that balance
is maintained on the fuel coming in the bowl and the fuel used by the engine.
FIGURE 11. CONTROLLING FUEL BOWL PRESSURE.
A vent is located in the top of the carburetor bowl. This vent is needed so that air pressure is always present
on fuel in the carburetor bowl. (On military vehicles, this vent is usually connected to the inside of the
carburetor air horn or to the air cleaner.)
When the throttle is almost closed, the amount of air flowing through the carburetor bore is so small that
practically no vacuum is created in the venturi. Therefore, no fuel flows from the nozzle in the venturi.
The low-speed circuit supplies fuel under this condition.