within it. From this point, the fuel is conducted to the
The high-speed and low-speed circuits overlap. As
nozzle, extending into the venturi. As the throttle valve
the throttle is moved from closed to opened position,
is opened, its linkage raises the metering rod in the jet.
for example, the idle- and low-speed circuit and the
The rod has several steps, or tapers, machined on the
high-speed circuit successively take over the main job
lower end and, as it is raised in the jet, it makes the
of supplying fuel. With a closed throttle, the idle
effective size of the fuel orifice greater, permitting more
discharge hole alone supplies fuel. As the throttle is
fuel to flow through the circuit to meet the load demand
opened so its edge moves past the low-speed hole, the
imposed upon the engine. At the wide-open throttle
low-speed hole begins to discharge fuel to meet the
position, the smallest step of the metering rod is in the
added fuel requirements of the engine as engine speed
circular opening of the jet, permitting the maximum
increases. As the throttle opens still wider, the
amount of fuel to flow through the circuit to meet the
additional and faster moving air through the air horn
requirements of maximum power. The metering rod
begins to produce fuel discharge from the main nozzle.
position must be synchronized with every throttle valve
As the main nozzle takes over, the low-speed discharge
position so that the proper ratio of air and gasoline is
hole no longer functions.
delivered to the engine for all speeds and driving
The carburetor is so designed that the fuel-air ratio
conditions.
of the mixture supplied with an open throttle provides
The vacuum step-up (fig. 4-16) operates much like
economical operation. This ratio is approximately 15:1,
the power jet. It consists of a step-up piston that is
by weight.
fastened to a step-up rod. When high vacuum develops
If the mixture is enriched to approximately 11:1 or
in the intake manifold, as it does under part-throttle
12:1, the engine will produce greater power even
operation, atmospheric pressure holds the step-up
though it will not run quite as economically. To increase
piston down against its spring pressure so that the
engine power for high-speed or heavy-load operation
step-up rod is held down in the step-up jet, thus closing
(as when pulling an aircraft up a hill), and still maintain
off the jet. With wide-open throttle, there will be a low
reasonable economy, an additional circuit is included in
vacuum in the intake manifold and the difference in
the carburetor that enriches the mixture with a
pressure above and below the piston is small.
wide-open throttle. This is known as the full-load,
Consequently, the piston is moved up by its spring
high-speed circuit, and it may be of several types.
pressure and the rod is raised out of the jet. Additional
In some carburetors, a power jet provides the
fuel for full-power operation is supplied.
additional fuel necessary for maximum power at
wide-open throttle. The power-jet valve (fig. 4-14),
controlled by a vacuum-actuated piston assembly,
STEP-UP PISTON ASSEMBLY
operates in accordance with the throttle opening. With
the throttle closed, a high manifold vacuum is present,
STEP-UP PISTON
SPRING
and the vacuum-controlled piston assembly is moved
by atmospheric pressure in the float chamber to the top
STEP-UP
of its cylinder against the tension of a spring, closing
ROD
the valve. When the throttle is opened to a point where
additional fuel is required, the manifold vacuum has
decreased sufficiently so that the spring on the piston
STEP-UP
VENT
assembly moves the piston down, thereby opening the
JET
TUBE
power jet to feed additional fuel into the high-speed
circuit. The jet valve is sometimes referred to as the
MAIN METERING
economizer valve, since its operation provides
JET
full-power operation where needed and more
economical operation at other times.
VACUUM PASSAGE
Instead of using a power jet, some carburetors
accomplish the same result by employing a metering
rod (fig. 4-15), which varies the size of the high-speed
ASf04016
jet openings. Fuel from the float bowl is metered to the
high-speed circuit through the calibrated orifice
provided by the high-speed jet and the metering rod
Figure 4-16.--Vacuum set-up high-speed circuit.
4-10
