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PART 10-5 MOTORCRAFT MODEL 4300 CARBURETTOR
10-51
throttle opens slightly
ahove
an
idle
<'ondition.
the whole length
of
the
slot
becomes
a
discharge port to richen
the mixture
(Fig.
6). This
secon'
d ary
discharge
opening
prevents the in-
crease
in
air
flow from making the
mixtUie too lean. There would be
a
"flat spot"
in the
transition
from the
idle to the main metering
system
without
the transfer
slot.
PRIMARY MAIN
FUEL
METERING
SYSTEM
A primary main fuel metering
sys-
tem, divided into two parts _
one
for
each
primary barrel _
provides
the fuel required by the
engine
at
cruising
speeds.
Main metering
sys-
tems
are calibrated
to deliver a lean
mixture _about 15 parts air to one
part
gasoline
_
when the
engine
is
cruising under light load. When more
power
is
required. the main metering
system continues
to
operate,
and the
mi¥ture is made richer by other
sys-
tems.
Parts of the Main Metering System
In the Model 4300 carburettor,the
primary main metering
system
(Fig.
7' has two main metering jets; main
wells. and main well tubes;
calibrated
air bleeds; disC'harge nozzles; and
booster venturis. At rest, fuel flow!;
from the bowl, through the main jets,
and into the main wells and
tubes.
In
each
well and tube, the fuel assumes
the
same
level as in the bowl until
the
engine
begins to operate.
Pressure Difference Causes Flow
With the
engine
operating, the
main metering
system
delivers fuel
in response to the throttle plate
opening.
Opening the throttle causes
air flow through the main
venturi
and booster venturi; the flow through
OAIR
FUEL
(;11
FUEL AND
AIR
VACUUM
the booster venturi
causes
a pressure
drop or
partial
vacuum at the dis-
charge
nozzle.
The fuel howl is at air
horn pressure, resulting in a preSSJ.Ire
difference that creates flow through
the
system.
Fuel is sprayed out the
discharge nozzle and mixes with the
airstream.
The size of the main jet determines
how much fuel is delivered for a
given
volume of air flow. Increasing or de-
creasing throttle opening increases or
decreases the fuel delivery
so
that
the mixture proportion or ratio is
quite
constant.
Bleed Assists Vaporization
The high-speed air bleeds (Fig.
!!)
in the
system
permits
some
air to be
mixed with the fuel in the main well.
The air
enters
the main well tube
through two holes when fuel is flowing
in the
system.
Adding air at this
point assists vaporization, and com·
pensates for the tendency
of
the
air
to become less dense at high
speeds.
The bleed also doubles as an anti-
siphoning
vent at low
speeds.
And it
discourages percolation when a hot
engine
is
shut
down by
venting
the
main well
.
ACCELERATING PUMP SYSTEM
Air,
being
very
light, responds
rapidly to
changes
in the throttle
opening.
Gasoline
is
heavier, and
therefore not as responsive. When the
throttles are opened
suddenly,
air
flow increases rapidly, but fuel flow
lags.
So
that the
engine
will respond
instantly to
opening
the throttle,
an
accelerating pump system.
which
furnishes
1' 1
.
single spurt of
fuel in
each
primary
venturi
when
the
throttles
are opened,
is
incorporatC'd
.
OAIR
FUEL
1[;1
FUEL AND AIR
VACUUM
MAIN
METERING
JET
Piston-Type Pump
A
piston-type
pump
(Fig.
9) is
actuated by a link from the accele-
rator linkage and by a spring to cause
the pumping action. The pumping
cham
her is formed below a
cup on
the
pump piston. A hall-type intake
check
valve
and a needle-type dis-
charge
valve
control the flow of fuel
into and
out
of the .pumping chamber
and
channels.
Discharge
nozzles
open into both primary venturis.
Fuel Intake
Fuel intake occurs as the throttles
are closed (Fig.
9).
The accelerating
pump link pulls the piston up, com-
pressing the piston ring. A partial
vacuum
is created helow the piston
cup
in the pumping chamber.
Fuel in the bowl, at this time, is
exposed
to full atmospheric pressure.
as
the external vent valve lever also
is
actuated by the accelerating pump
link to open the
valve.
The pressure difference pushes the
intake check
valve
off
its seat and
causes
fuel to flow from the bowl into
the pumping
chamber.
The discha'rge
valve
is
seated.
and prevents backflow
in the discharge
passages.
J:'uel
Discharge
When the throttles open (Fig. 10),
the
end
of the accelerating pump link
moves down in the piston arm
slot,
and the spring pushes the piston into
the
oumoine
chamber.
PressurP.
builds up
~n
the
chamber
to force
the inlet
valve
closed
on
its seat.
Fuel is pumped through the dis-
charge
passages __ the discharge
·valve
is forced
open
hy the fuel pres-
sure
_
and
fuel is
spraY,ed out
the
discharge
nozzle.
When the piston
has
reached
its
limit of
travel
(depending
on
how far the accelerator
is depressed), flow
stops
and the dis-
charge valve seats.
The discharge
passages remain primed, or full of
fuel,
so
that pumping action through
the nozzles is instantaneous on the
next cycle.
BOOSTER
VENTURI
MAIN WELL TUBE
Fig. 6
·-
Idle
Transfer Slot is Secondary
Discharge Passage
Fig. 7- Primary
Main
Metering
System
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