The fuel flows through the main jet that meters
the amount of fuel entering the system. The fuel then
flows into the main discharge tube and emulsion tube.
The emulsion tube causes air from the air bleed
to mix with the fuel. The fuel, mixed with air, is finally
pulled out the main nozzle and into the engine.
The full-power system provides a means of
enriching the fuel mixture for high-speed, high-power
conditions. This system operates, for example, when
the driver presses the fuel pedal to pass another vehicle
or to climb a steep hill. The full-power system is an
addition to the high-speed system. Either a metering
rod or a power valve (jet) can be used to provide
variable, high-speed air-fuel ratio.
A metering rod is a stepped rod that moves in and
out of the main jet to alter fuel flow. When the rod is
down inside the jet, flow is restricted and a leaner fuel
mixture results. When the rod is pulled out of the jet,
flow is increased and a richer fuel mixture results for
more power output. The metering rod is either
mechanical-linkage or engine-vacuum operated.
The MECHANICAL LINKAGE metering rod
(fig. 4-26) is linked to the throttle lever. Whenever the
throttle is opened wide, the linkage lifts the metering
rod out of the jet. When the throttle is closed, the
linkage lowers the metering rod into the jet.
The VACUUM OPERATED metering rod (fig.
4-27) that is controlled by engine vacuum is connected
to a diaphragm. At steady speeds, power demands are
low and engine vacuum is high, and the piston pushes
the metering rod into the jet against spring pressure,
restricting the flow to the discharge tube. When the load
increases, vacuum decreases, causing the piston spring
to lift the metering rod out of the jet, progressively
increasing the flow of fuel to the discharge tube.
Figure 4-27.Vacuum operated metering rod.