Ford COUGAR 1968 Shop Manual page 538

GROUP 10
-
Fuel
System
When the throttle plate is progres-
sively opened, engine speed increases
and the air passing through the boost-
er venturi gradually creates sufficient
vacuum to bring the main fuel sys-
tem into operation and fuel will be
discharged through the main dis-
charge nozzle. The difference in pres-
sure between the primary venturi
and the fuel bowl causes fuel to flow
through the primary stage main fuel
system (Fig.
8). '
The fuel moves up the main well
past the main well air bleeds in the
side of the well. Filtered air enters
through the main air bleed in the main
body and then into the metering block
by interconnecting passages. This mix-
ture of fuel and air, being lighter
than raw fuel, responds faster to any
change in venturi vacuum and atom-
izes more readily when discharged
into the air stream of 'the venturi.
The mixture of fuel and air moves
up the main well passage in the
metering block and passes into the
short horizontal passage leading t o the
main body, then through the horizon-
tal channel of the discharge nozzle.
The fuel is discharged into thebooster
venturi where it is atomized and mix-
ed with air flowing through the car-
buretor.
POWER FUEL SYSTEM
The primary power fuel systems
for the primary and secondary carbu-
retors operate in the same manner;
therefore. only one carburetor is dis-
cussed.
During periods of increased road
loads or high power operation, the
carburetor has a tendency to lean out
as the air flow is increased. T o sup-
plement the primary stage main fuel
system, additional fuel is required to
maintain the proper fuel-air ratio. The
added fuel required during this period
is supplied by the power fuel system
(Fig. 9).
The power fuel system is controlled
by manifold vacuum, which gives an
accurate indication of the power de-
mands placed on the engine. Manifold
vacuum is highest at idle speed and
decreases as the load on the engine is
increased.
Manifold vacuum is t r a n s m i t t e d
from an opening in the base of the
throttle body, through a passage in the
throttle bokly, and main body to the
power valve chamber in the primary
metering block. The manifold vac-
uum, acting on the diaphragm a t
idle or normal load conditions. is
strong enough to overcome the ten-
@.FuEL.AtR
,
SECONDARY THROTTLE
OPERATING DIAPHRAGM HOUSING
\
SECONDARY OPERATING L E V E R
A t & V A C U U M
B1408-C
FIG.
10-Typical
Holley 4-V S e c o n d a r y S t a g e Main Fuel System
sion of the power valve spring and
holds the diaphragm closed. When
high power operation places a great-
er load on the engine and manifold
vacuum drops below a predetermined
value, the power valve spring over-
comes the reduced vacuum and opens
the power valve. Fuel flows through
the power valve and through the diag-
onal restrictions in the metering
block into the main well. In the main
well, the fuel joins the fuel flow from
the primary stage main fuel system,
enriching the mixture.
As engine power demands are re-
duced, manifold vacuum increases.
The increased vacuum overcomes the
tension of the power valve spring and
draws the valve diaphragm closed.
T h ~ s closes the power valve and shuts
off the added fuel supply.
'
SECONDARY STAGE
THROTTLE OPERATION A N D
.
M A I N FUEL SYSTEM
The secondary stage throttle oper-
ation and.main fuel systerhs for the
primary and secondary carburetors
are the same in operation; therefore,
only one carburetor is discussed.
The secondary stage of the carbu-
retor is supplied with fuel from the
secondary fuel bowl which receives
its fuel through a connecting tube at
the fuel inlet fitting on the primary
fuel bowl. The secondary fuel inlet
system operates the same as the pri-
mary fuel inlet system.
The secondary stage throttle plates
are operated by a spring-loaded vac-
uum diaphragm assembly attached to
the side of the main body and linked
to the secondary throttle shaft (Fig.
10).
Opening of the secondary throttle
plates is controlled by vacuum from
the right primary and secondary ven-
turis. A t high speeds. when engine
requirements approach the capacity
of the two primary barrels, the in-
creased primary venturi vacuum
moves the secondary diaphragm, com-
pressing the diaphragm spring. The
diaphragm, acting through the dia-
phragm link and lever, starts to
open the secondary throttle plates.
The position of the throttle plates de-
pends upon the strength of the vac-
uum. This, in turn, is determined by
the air flow through the primary bar-
rels. As the secondary throttle plates
begin to open, the
vacuum
in the
secondary barrels increases. The in-
creased vacuum in the right second-
ary venturi 'assists the primary ven-
turi vacuum in further opening the
secondary throttle plates. As top speed
is reached, the secondary throttle