Figure 1-26.Venturi effect.
Figure 1-27.Throttle valve.
of the throat is called the venturi (fig. 1-26). A tube
called a discharge nozzle is positioned in the venturi
and is connected to a reservoir of gasoline called the
The downward intake stroke of the piston creates a
partial vacuum in the carburetor throat that allows
low-pressure air to rush by the fuel nozzle. This forces
small drops of fuel to be mixed with the air. Then the
fuel and air mixture must pass the throttle valve which
is controlled by the operator (fig. 1-27). The throttle
valve opens or closes to allow the correct volume of the
fuel and air mixture into the engine. The choke valve
(fig. 1-28) also controls the supply of fuel to the engine.
When you start the engine in cold weather, the choke
valve can be partly closed, forming a restriction that
causes more fuel and less air to be drawn into the
combustion chamber. This results in a richer air to fuel
Figure 1-28.-Choke valve operation.