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PART 10-5 MOTORCRAFT MODEL 4300 CARBURETTOR
10-53
MAIN WELL
0
AIR
•
FUEL
VACUUM
VALVE OPENS
Fig. 12- Power Valve Action
venturis.
The following
deals
with
the
"secondaries",
the
other two ven-
turis that
give an extra volume of
fuel-air
mixture
when
maximum
power is applied.
The
secondary
throttles are linked
mechanically to
the
primaries, and
begin to
open
when the primariPs
are
three-q
uarters
open.
Air Valve Plates
Each
secondary venturi (barrel)
(Fig.
13) has
an offset air valve plate
located below
its
booster venturi
.
These plates are
preloaded
by a
spiral torsion
spring,
which holds
them closed when
the
secondary
throttles are closed
.
The air valve plates being
closed,
cause a vacuum
as soon as
the
throttles begin to
open.
The fuel
bowl
is at air horn
pressure.
The
pressure
difference
starts
fuel flowing from
the bowl; through the
seconda:y
main jet tube to the
secondary
roam
wells;
·
then through the main well
Fig. 14- Automatic Choke System
SECONDARY CHANNEL
BLEED
0
AIR
FUEL
8
FUEL AND AIR
VACUUM
ENRICHMENT
DISCHARGE TUBE
Fig. 13- Secondary Main Metering System
tubes
and
out the
enrichment
dis-
charge
tubes.
Secondary Main Discharge Tubes
A
second stage
of fuel
supply
occurs
when the air valve plates are
pulled
open
by manifold
vacuum
below
them.
Increased air flow then
causes
fuel to be discharged into the
booster venturis from the
secondary
main
discharge
tubes.
The
amount
that the
air valves
open, and
therefore
the amount of
mixture delivered,
is controlled by
the difference in
air
pressure
acting
on the
plates balanced against the
spiral
torsion spring. An integral
hydraulic
dashpot
dampens
sudden
movemPnts
of
the plates
to
prevent
flutter
and erratic
e
ngine
operation.
Air Bleeds
Air
is
introduced
into the
fuel at
the
secondary channel anti-siphon
hleed
and at
th
e
high
-speed
bleeds
to
help
vaporization.
The bleeds also
act as anti-percolation vents at idle
or
when a hot
engine
is
shut
down,
by relievi-ng pressure
in
the main
well
tubes.
AUTOMATIC CHOKE SYSTEM
To
start
and operate a
cold
engine,
it is necessary
to supply
a richer mix-
ture, because
the
fuel
doPsn't vapor-
ize as readily
and
because
part
of the
vaporized
fuel
condenses
inside the
cold
intake manifold. The
choke
sys-
tem, by regulating the position of the
choke
above the primary venturis,
provides the means
of
making the
cold
mixture
rich.
and gradually let-
ting it become leaner as the
engine
warms
up.
Thermostatic Spring Controls
Choke Plate
The thermostatic spring in the
choke housing
(Fig. 14)
exerts a
torque
that
forces the linkage to close
the
choke
plate when the engine is
cold. The choke plate is closed fully
FAST IDLE ADJUSTMENT SCREW
Fig. 15- Fast Idle Cam
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