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10-50
0
AIR
II
FUEL
Fig. 4-Fuel Inlet System
OPERATION
The Model 4300 4-V Carburet tor is
designed to supply a calibrated fuel-
air mixture to
a
V
-8
engine. In
normal operation, each of the prim-
ary venturis supplies all the fuel-air
mixture required by four cylinders.
The idle, main metering, power fuel
supply, accelerating pump and choke
systems go into
operation
automatic-
ally to provide the proper richness
or leanness of
the
mixture for the
operating condition.
Operation
of
the fuel metering sys-
tems is controlled by the accelerator
linkage, throttle position and engine
speed.
The
choke system
is controlled
by the throttle position and the tem-
perature of the
engine
exhaust mani-
fold.
FUEL INLET SYSTEM
Correct calibration of the carbu-
rettor depends
on fuel
being available
at a specific level
in
the fuel bowl.
If
the fuel level
is
low, the metering
systems
deliver Jean mixtures; if the
level is high, mixtures are rich. The
function
of
the fuel inlet
system
is
to admit gasoline into the fuel bowl
and maintain the
soecified
level.
Fig. 4
shows the construction
and
operation
of the fuel inlet
system.
The fuel inlet is constantly charged
with fuel under pressure from the
fuel pump. This fuel
enters
the bowl
through the fuel inlet
valve,
which is
permitted to open when the float
lowers.
The float moves up-and-down with
the fuel level. When
enough
fuel has
entered
to fill the bowl to the correct
level, the float
is
high
enough
for the
GROUP 10-
FUEL SYSTEM
SPECIFIED
FLOAT
O
AIR
•
FUEL
SPECIFIED
FLOAT
LEVEL
_j
FLOAT
DROPS
~
Fig. 5-Auxiliary Fuel Valve Operation
float lever to push the inlet valve
(needle) against its
seat.
Flow of the
fuel into the bowl then is blocked
until some fuel is used and the float
lowers
again.
Auxiliary Fuel Inlet Valve
An auxiliary fuel inlet valve is built
into this system to supplement the
main
or
primary fuel inlet valve when
engine fuel requirements are high.
The main or primary fuel inlet
valve controls small fuel flows pre-
cisely because of its small area of
opening and relatively high valve-to-
seat sealing pressure. When large
fuel flows are required, as in high
engine speeds· and heavy-load con-
ditions, the fuel level and float height
drop, thereby opening the auxiliary
valve
(Fig.
5) in addition to the main
or primary fuel inlet valve. The total
combined fuel valve opening is larger
than the previous single valve that
has been used in former Ford car-
buretors.
In addition to supplying fuel for
high engine load conditions, the large
combined
valve opening also purges
the
carburettor-to-fuel
rump line,
after a hot restart, of fue vapor that
forms during a hot soak condition.
Venting the Bowl
Two stand pipes beside the air horn
(Fi~.
4)
vent
the bowl to
the
fresh air
inlet.
The stand pipes are
open
to the
carburetor intake
air after the air
passes through the air cleaner. Thus,
the howl
pressure and air
horn
pres-
sure are equal during
main metering
system operation,
and
the calihration
o(
the carhuretor
isn't
affected by the
air cleaner's condition.
IDLE FUEL SUPPLY SYSTEM
When
the throttles are closed or
nearly closed, there is not
enough
air
flow
through the venturis
to
create
the vacuum needed
to operate
the
primary
main
metering
system.
Therefore,
a separate
fuel metering
system for idle operation
is
incorpo-
rated.
The primary idle
fuel supply sys-
tem (Fig. 6) uses
the pressure
dif-
ference between manifold
vacuum
and atmospheric pl'essure in the
bowls to cause fuel flow. Idle
system
fuel flow is from the bowl, through
the main metering jets
and into
the
main wells. From there, the fuel flows
up through
calibrated
restriction in
the idle
tubes, then down the
idle
channels
to the idle
cavities in .the
throttle body. It
enters
the
venturis
below the throttle plates through
the
idle discharge port
and
idle transfer
slot. The idle fuel adjustment
screw
r,
e gulates
the amount
of
fuel that is
discharged through the
port.
Air Bleeds
Filtered air is mixed with the fuel
through the idle
air bleeds to
help
the fuel
atomize
as it is discharged.
The bleed also prevents
siphoning
through the idle
system at very
high
speeds, or
when
the engine
is
shut
down.
Idle Transfer Slot
The idle transfer
slot in each
ven-
turi
serves both as an air
bleed
and
as a
secondary discharge
port. At
closed throttle
(Fig.
6),
the
top of
the
slot
admits
air
mto the idle
cavity, and
the bottom
of
the
slot
delivers
fuel
to
the
venturi.
When the
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