tilted at various angles in the carburetor throttle valve
through which the air must pass on its way to the intake
body. When it is parallel to the airflow, it offers almost
manifold and the engine cylinders. The purpose of the
no restriction, and a full volume of fuel-air mixture can
venturi is to create a partial vacuum (pressure less than
pass into the intake manifold. The throttle valve is
atmospheric) at the outlet of the fuel nozzle. This will
connected by linkage to the control (accelerator pedal
permit the atmospheric pressure on the surface of the
or hand throttle) in the driver's compartment/operator
gasoline in the float bowl to force the gasoline out
through the nozzle. This gasoline then sprays and
atomizes in the passing air to form the fuel-air mixture.
Several controls and circuits must be added to the
simplified carburetor, shown in figure 4-10, for it to
The float circuit maintains the fuel level in the
vary the fuel-air ratio for different operating conditions.
carburetor bowl at a constant level during engine
These controls and circuits include:
operation (fig. 4-12). The float circuit permits fuel to
· The accelerating system, which momentarily
flow into the bowl when the fuel level drops below a
predetermined level, and shuts off the flow of fuel when
the level reaches the specified height. Up-and-down
when the throttle is opened.
movement of the float controls the fuel supply by
· The maximum power circuit, which enriches the
means of a needle valve and seat.
mixture when the throttle is opened wide for
The float level must be set with accuracy in ac-
high-speed or full-speed operation.
cordance with the manufacturer's specifications. If the
· The float system, for keeping a constant level of
fuel level is too low, insufficient fuel will be supplied to
gasoline in the carburetor bowl.
the jets, and engine performance will be sacrificed. (A
jet is an orifice through which fuel is metered to the fuel
· The idling circuit, for supplying an enriched
nozzle.) On the other hand, if the fuel level is too high,
mixture during engine idling.
excessive fuel will reach the jets. In fact, fuel may
· The choke circuit for engine starting.
continue to flow from the jets when the engine is
stopped. In actual operation, the float and needle valve
maintain a position that permits the fuel coming in to
just balance the fuel that is flowing to the jets.
The throttle is a device in the carburetor that is used
to vary the amount of fuel-air mixture that enters the
intake manifold. This is necessary so the speed of the
engine can be controlled. The throttle valve (fig. 4-11)
When the throttle is almost closed, there will be
is simply a round disc mounted on a shaft so it can be
very little air passing through the venturi. Therefore,
there will be very little vacuum at the venturi, and the
fuel nozzle will not discharge any appreciable amount
of fuel. Without some additional circuit to assure fuel
delivery with a closed throttle, the engine would stop.
The circuit that takes care of fuel delivery during closed
or nearly closed throttle is called the idle and low-speed
N E E D L E VA LV E
F L O AT
VA LV E
Figure 4-11.--Simplified carburetor with throttle valve.
Figure 4-12.--A typical float circuit.