Buick LeSabre 1965 Service Manual page 128

3 - 5 2 4-BARREL CARTER
ENGINE FUEL AND EXHAUST
The position of the step-up rod in
the primary main metering jet
controls the amount of fuel ad-
mitted to the n o z z l e s . The
position of the step-up rod is
controlled by a manifold vacuum
piston.
During normal part throttle op-
eration, manifold vacuum pulls
the step-up piston and rod as-
sembly down, holding the larger
diameter of the step-up rod in
the primary main metering jet.
This is true when the vacuum
under the piston is strong enough
to overcome the tension of the
step-up piston spring. Fuel is
then metered around the larger
diameter of the step-up rod in
the jet.
Under any operating condition,
when the tension of the spring
overcomes the pull of vacuum
under the piston, the step-up rod
will move up so its smaller diam-
eter or power step is in the jet.
This allows additional fuel to be
metered through the jet. The
step-up rod does not require
adjustment.
Fuel for the high-speed circuit
of the secondary is metered at the
main metering jets (no step-up
rods used). See Figure 3-68.
Initial discharge ports are incor-
porated to supplement starting of
the fuel flow in the secondary
high-speed circuit. These ports
are located next to the venturi
struts. When the auxiliary valves
start.to open, the vacuum on the
initial discharge ports pulls fuel
from the main vent tube well
through passages that rise above
the fuel level in the bowl. Air
bleeds serve to break-up the liq-
uid fuel and mix it with air as it
moves through the passages to the
initial discharge ports where it is
discharged into the air stream.
As the auxiliary valves continue
to open, and the secondary
nozzles start delivering fuel, less
fuel flows from the initial dis-
charge ports.
AIR BLEED-,
HIGH SPEED
AIR BLEED *|
1 VI
pNITIAL 1
IDISCHARGES-
IPORT ~ t
I MAIN H
• VENT—B
I TUBE •
i
MAIN METERING_\
JET
SECONDARY SIDE PRIMARY SIDE |
^ |
r
NOZZLE
<
1
H B . AUXILIARY
• mT~ VALVE
5)1
Figure 3-68—Secondary High
Speed Circuit
The main vent tubes on primary
and secondary sides mix air
drawn through the high speed air
bleed with the fuel before it
passes out of the nozzles.
Air bleeds in the primary nozzle
passage plugs cause the air fuel
mixture to closely follow the re-
quirements of the engine. Also,
their position at the junction of
the nozzle passage and the main
vent tube serves to break the flow
of fuel from the high speed circuit
quickly as the throttle is closed
upon deceleration.
A clogged air bleed or main vent
tube may cause excessively rich
mixtures. The high speed bleed
and main vent tubes are perma-
nently installed. If replacement
is necessary, use a new venturi
assembly.
The high speed bleeds also act as
anti-percolator vents when a hot
engine is stopped or at idling
speed. This will help vent fuel
vapor pressure in the high speed
and idle well before it is suf-
ficient to push fuel out of the
nozzles and into the intake
manifold.
Engines operated at part throttle
on level road use a mixture of
maximum leanness. The mixture
for greatest power and accelera-
tion is somewhat richer, and is
furnished by the power and accel-
erating systems described later.
The high speed systems in the
primary section control the flow
of fuel during the intermediate or
part throttle range of operation
and up to approximately 85 MPH.
The secondary throttle valves re-
main closed until the primary
valves have opened approximately
50-55 degrees, after which they
are opened proportionately so that
all valves reach the wide open
position at the same time. While
the secondary valves are closed,
the Auxiliary valves located above
them a r e held closed by the
weights on the auxiliary valve
shaft lever (Figure 3-76); there-
fore there is not sufficient air
flow through the barrels to oper-
ate the high speed systems in the
secondary section.
When the secondary t h r o t t l e
valves are open and the engine
speed is at least 1400-1600 RPM,
the resulting air flow through the
secondary barrels starts to open
the auxiliary valves because their
supporting shaft is located off-
center in the barrels. When the
auxiliary valves are open the high
speed systems in the secondary
section also supply fuel to the
engine.
e. Operation of the
Power System
For maximum power or high
speed operation above approxi-
mately 85 MPH, a richer mixture
is required than that necessary
for normal throttle opening. The
r i c h e r mixture is s u p p l i e d
through the high speed systems
in the primary section through
vacuum control of the step-up
rods.
Each power circuit consists of a
vacuum piston located in a cyl-
inder connected to manifold vac-
uum and a spring which tends to
push the piston upward against
m a n i f o l d vacuum. See Figure
3-69.
Under part throttle operation,
manifold vacuum is sufficient to