Buick LeSabre 1965 Service Manual page 109

ENGINE FUEL AND EXHAUST SYSTEMS 2-BARREL ROCHESTER 3 - 3 3
and low speed operation of the
engine.
The idle system consists of the
idle tubes, idle passages, idle
air bleeds, idle a d j u s t m e n t
needles, off-idle discharge slots
and the idle adjusting needle
holes.
In idle speed position, each throt-
tle valve is slightly open, allowing
a small amount of air to pass be-
tween the wall of the carburetor
bore and the edge of the throttle
valve. Since there is not enough
air flow for venturi action, the
fuel is made to flow by the appli-
cation of vacuum (low pressure)
directly through the idle system
to the fuel in the carburetor bowl.
See Figure 3-28.
Fuel from the float bowl passes
through each main metering jet
into the main well where it is
metered by the orifice at the
lower end of the idle tube. It then
passes up the idle tube and is
mixed with air at the top of the
idle tube by two calibrated idle
air bleeds. The air/fuel mixture
then passes down through a cali-
brated restriction into a vertical
passage past a third idle bleed to
the idle port located just above
each closed throttle valve. Here
the mixture is again bled with air
and then moves down to the idle
needle hole where it combines
with air by-passing the slightly
open throttle valve. The idle mix-
ture needle controls the amount
of fuel mixture which enters the
carburetor bore at curb idle po-
sition of the throttle valve.
As the throttle valve is opened
further, more and more of the
idle port is exposed to manifold
vacuum. This port supplies addi-
tional fuel mixture for off-idle
engine requirements.
On all air conditioner equipped
cars, a special thermostatic air
valve is added in the hole in the
rear side of the throttle body.
This valve is designed to com-
pensate for loss of engine RPM
while idling under very hot oper-
ating conditions. When the under-
hood temperature rises beyond a
certain point, the calibrated ther-
mostatic spring opens the valve.
This allows additional air to flow
in below the throttle valves. At
normal operating temperatures,
the valve should be closed. The
valve cannot be adjusted or re-
paired; therefore, a faulty valve
must be replaced.
d. Operation of Main
Metering (High Speed)
System
As the throttle valve continues to
open, the edge of the throttle
valve is gradually moved away
from the wall of the carburetor
bore, reducing the vacuum so that
the discharge of fuel mixture at
the idle needle hole and off-idle
port gradually diminishes.
With the increased throttle open-
ing, there is increased velocity
in the venturi system. T h i s
causes a drop in pressure in the
large venturi which is increased
many times in the small venturi.
Since the low pressure (high vac-
uum) is now in the small venturi,
fuel will flow in the following
manner:
Fuel from the float bowl passes
through the main metering jets
into the main well and rises in the
main well tubes. Air entering the
main well through the main well
bleeds is mixed with fuel through
calibrated holes in the main well
tube. The mixture then moves up
and out of the discharge nozzle
into a channel where more air is
added. The mixture travels down
through the channel to the small
venturi where it is delivered to
the air stream and then to the
intake manifold. See Figure 3-29.
e. Operation of Power System
To achieve the proper mixtures
required when more power is de-
sired or for extreme high speed
driving, a vacuum operated power
piston in the air horn and a power
valve located in the bottom of the
float bowl are used. Through a
connecting vacuum passage from
the base of the carburetor to the
power piston cylinder in the air
horn, the power piston is exposed
to manifold vacuum at all times.
See Figure 3-30.
During idle and part throttle op-
eration, the relatively high vac-
uum holds the power piston up
against spring tension-and the
power valve remains closed.
Increase in engine load lowers
the manifold vacuum. When it has
dropped sufficiently the power
piston spring overcomes the up-
ward vacuum pull and the power
piston moves downward, opening
the power valve to allow addition-
al fuel to flow through calibrated
restrictions into the main well.
As the engine load decreases, the
resulting higher vacuum over-
comes the spring tension on the
power piston and raises the power
piston closing the power valve.
A 2-stage power valve is used.
In the first stage, fuel is metered
by the valve itself. This stage is
used for light power loads. On
heavy power loads the valve is
fully opened to the second stage,
and in this location the power
valve allows the fuel to be me-
tered by the power restrictions in
the fuel channel located in the
bottom of the fuel bowl.
It will be noted that the power
piston cavity in the carburetor air
horn is connected to the main air
flow passage by a vacuum relief
passage. It is the purpose of this
passage to prevent the transfer of
vacuum acting on the piston from
acting also on the top of the fuel
in the float bowl. Any leakage of
air past the upper grooves of the
piston will be compensated for by
this relief passage and will not
affect carburetor metering.