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:
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.
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.
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.
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.
system operation is as follows:
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.
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.
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.
by SweetHaven Publishing Services
Based upon a text provided by the U.S. Navy