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3-28
CARTER CARBURETOR
ENGINE FUEL AND EXHAUST SYSTEMS
(102)
through the air bleed from the throat of the
carburetor. This additional air tends to break
the fuel particles into a still finer vapor.
The fuel-air mixture that flows downward
through the idle mixture passage and out
through the two idle ports is still richer than
an idle mixture needs to be, but when it mixes
with the air coming in past the throttle valve,
it forms a combustible mixture of the right
proportions for idle speed. The idle adjustment
screw permits regulation of the quality of the
low speed mixture.
The upper idle port is slotted vertically. As
the throttle valve is opened it not only allows
more air to come in past it but also uncovers
more of the idle port, thereby allowing a greater
quantity of the fuel-air mixture to enter the
carburetor throat from the. idle mixture pas-
:
sage.
The closed position of the throttle valve is
such that at idle speed of 8 to 10 MPH, it
leaves enough of the slotted idle port in reserve
to cover the range in speed between idle and
the point where the high speed system begins
to operate.
As the speed increases from approximately
20 MPH, the low speed system starts cutting
out as the high speed system cuts in until the
high speed system is carrying the entire load
and the low speed system is doing nothing.
METERING
ROD
PISTON
SPRING--"'---=~
METERING
ROD JET
Fig. 3-35-High Speed and Power Systems-Carter Carburetor
d. Operation of Carter High Speed System
The high speed system controls the flow of
fuel during the intermediate or part throttle
operation, starting at approximately 20 MPH
and continuing up to approximately 75 MPH.
The operation of the high speed system in
each barrel of the carburetor is identical. Air
entering the barrel through the air horn passes
through the triple venturi system which in-
creases the velocity of the air and creates a
suction on the main nozzle. This causes fuel
to flow from the float chamber through the
metering rod jet into the main nozzle from
which it is discharged into the air stream pass-
ing through the small venturi. The triple ven-
turi system tends to atomize or break up the
fuel into a vapor and mix it with the air
stream. See figure 3-35.
.
If any vapor bubbles are formed in the hot
gasoline in the main nozzle passage, they rise
in the low speed jet well and the vapor ex-
hausts through the anti-percolator passage into
the main nozzle. This avoids percolating diffi-
culties which might occur if the vapor bubbles
rose directly into the main nozzle.
The amount of fuel entering the high speed
system is metered or controlled by the area of
the opening between the metering rod jet and
the end of the metering rod which extends into
the jet. The lower end of the metering rod has
steps of three different diameters to provide
three different metering areas, depending upon
the position of the metering rod in the jet. The
metering rod is connected by a link, counter-
shaft and connector rod to the throttle shaft so
that it is raised when the throttle valve is
opened and lowered when the throttle valve is
closed.
At approximately 20 MPH the largest or
economy step of metering rod extends into the
jet, thereby giving the smallest possible meter-
.ing area. As the throttle valve is opened for
higher speed or greater power, the metering
rod is raised so that the middle step and later
the smallest or power step provides increased
metering area between rod and jet. At top
speed, the smallest or power step is in the jet.
Engines operated at part throttle on level
road use a mixture of maximum leanness. The
mixture for greatest power and acceleration is
somewhat richer, and is furnished by the power
and accelerating systems described below.
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