Understanding each of these systems is important.
It will help you when diagnosing and repairing
The float system (fig. 4-20) maintains a steady
working supply of gasoline at a constant level in the
carburetor. This action is critical to the proper operation
of the carburetor. Since the carburetor uses differences
in pressure to force fuel into the air horn, the fuel bowl
must be kept at atmospheric pressure. The float system
keeps the fuel pump from forcing too much gasoline
into the carburetor bowl. An excessively high float
level will cause fuel to flow too freely from the
discharge tube, causing an overly rich mixture, whereas
an excessively low float level will cause an overly lean
mixture. The basic parts of the float system are the fuel
bowl, the float, the needle valve, the needle seat, the
bowl vent, and the hinge assembly. Study the
relationship of each part as follows:
The CARBURETOR FLOAT rides on top of the
fuel in the fuel bowl to open and close the needle valve.
It is normally made of thin brass or plastic. One end of
the float is hinged to the side of the carburetor body and
the other end is free to swing up and down.
The NEEDLE VALVE regulates the amount of
fuel passing through the fuel inlet and the needle seat.
The needle valve is usually made of brass. Sometimes
the end of the valve will have a soft viton (synthetic
rubber) tip. The soft tip seals better than a metal tip,
especially if dirt gets caught in the needle seat.
The NEEDLE SEAT works with the needle
valve to control fuel flow into the bowl. It is a brass
fitting that threads into the carburetor body.
The BOWL VENT prevents pressure or vacuum
buildup in the carburetor fuel bowl. Without venting,
pressure could form in the bowl, as the fuel pump fills
the carburetor. This could also cause vacuum to form in
the bowl, as fuel is drawn out of the carburetor and into
the engine. On vehicles equipped with an evaporation
control type emission system, the fuel bowl is vented
into a hose going to a charcoal canister instead of the
outside. The canister stores toxic fuel vapors and
prevents them from entering the atmosphere.
Basic float system operation is as follows:
When engine speed or load increases, fuel is
rapidly pulled out of the fuel bowl and into the venturi.
This action causes the fuel to drop in the bowl. The
needle valve also drops away from its seat. The fuel
pump can then force more fuel into the bowl.
As the fuel level in the bowl rises, the float
pushes the needle valve against its seat. When the fuel
level is high enough, the float closes the opening
between the needle valve and the seat by the rising float,
as the fuel reaches the desired level in the fuel bowl.
With the engine running, the needle valve usually
lets some fuel leak into the bowl. As a result, the float
system maintains a stable quantity of fuel in the bowl.
This is very important because the fuel level in the bowl
can affect the air-fuel ratio.
Figure 4-20.Float system.