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10-38
MAIN FUEL SYSTEM
As engine speed increases, the air
passing through the booster venturi
creates a vacuum. The amount of
vacuum is determined by the air flow
through the venturi, which in turn is
regulated by the speed of the engine.
The difference in pressure between
the main discharge port and the fuel
bowl causes fuel to flow through the
maio fuel system (Fig.
7).
At a predetermined venturi vacu-
um,
fuel flows from the fuel bowl,
through the maio jets, and into the
bottom of the main well. The fuel
moves up the main well tube past air
bleed holes. Filtered air from the
high speed air bleed enters the fuel
flow in the main well tube through
holes in the side of the tube
.
The high
speed air bleed meters an increasing
amount of air to the fuel as venturi
vacuum
increases,
maintaining the re·
quired fuel-air
ratio.
The mixture of
fuel and air is lighter than raw fuel
and responds faster to changes in
venturi
vacuum.
It also atomizes
more readily tha-n raw fuel. The fuel
an.
i
air continue up the main well
tube past another air bleed which
also acts as a vent to prevent siphon-
ing when the engine is shut down.
The fuel is discharged into the boost·
er venturi where it is atomized and
mixed with air flowing through the
carburettor.
The throttle plate controls the
amount of the fuel-air mixture
ad-
mi_
t ted to the intake manifold, regu-
latmg the speed and power output of
the engine.
A balance tube is located in each
barrel directly below the booster ven-
turi. When
decelerating,
the balance
tube siphons off any excess fuel drop-
lets remaining around the edge of the
booster venturi and discharges the
droplets into the equalizing slots in
the base of the carburetor where they
are mixed with the idle fuel. The
balance tube also acts as an addi-
GROUP
10-
FUEL SYSTEM
<;::J
AIR
<;:::J
FUEL- AIR
. . FUEL
<II)Im
VACUUM
POWER VALVE VACUUM PICK-UP
FIG. 8-Power Fuel System
tiona) air bleed during the idle fuel
system operation.
POWER
FUEL SYSTEM
During periods of increased road
loads or high speed
operation,
the
fuel-air ratio must be increased for
added power.
The
added fuel re-
quired during this period is supplied
by the power fuel system
(Fig.
8).
The power fuel system is con-
trolled by the intake manifold vac-
uum.
Manifold
vacuum is transmitted
from an
opening
in the
base
of the
main body, through a passage in the
main
body
and
power
valve chamber
to the
power
valve diaphragm. The
manifold vacuum, acting on the pow-
er valve at idle speed or normal road
82603-A
load conditions, is great enough to
hold the power valve diaphragm
down, overcoming the tension
of the
spring on the valve stem and holding
the valve
closed.
When high
power
operation places a greater load on
the engine and manifold vacuum
drops below a predetermined value,
the spring opens the power
valve.
Fuel
from
the
fuel
bowl
flows
through the power valve and into
passages leading to the maio fuel
well. Here the fuel is added to the
fuel from the main fuel system, en·
riching the mixture.
As engine power demands are re·
duced, manifold vacuum increases.
The increased vacuum overcomes the
tension of the
valve_
stem spring and
closes the power valve.
EJ
IN
CAR ADJUSTMENTS AND REPAIRS
CARBU!tETTOR ADJUSTMENTS
The
adjustments
described
and
illu strated
in
this
part
should
be
performed
JS
required
to
retain
the
desired
engine
performance characteristics. Refer
to the specifications whenever carburettor
adjustments
are
made.
IDLE SPEED AND FUEL
MIXTURE
All carburettors are equipped with idle
fuel
mixture adjusting
limiters.
The
limiters control the maximum idle richness
Jnd help prevent unauthorized persons
from making overly ri<.:h idle adjustment.
The plastic idle limiter cap is installed
on the head of the idle fuel mixture
adjusting screws), (Fig. 9). Any ad-
justment made on carburettors having
this type of limiter must be within the
runge of the idle adjusting limiter. Under
no circumstances are the idle adjusting
limiters or the limiter stops on the
carburettor to be mutilated or deformed to
render the limiters inoperative On the
Motorcraft Model
2100-D 2-V
carburettor,
the power valve cover must
be
installed
with the limiter stops on the cover in
position to provide a positive stop for tabs
on the idle adjusting limiters (Fig.
9).
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