Buick LeSabre 1965 Service Manual page 116

3 - 4 0 4-BARREL ROCHESTER
ENGINE FUEL AND EXHAUST
-INTERNAL VENTS
BOWL BALANCE
CHANNEL
FLOAT-
NEEDLE
SEAT
FLOAT
NEEDLE'
Figure 3-41—Primary and Secondary Float Systems
When the fuel is used from either
the primary or secondary side of
the float bowl, the floats auto-
matically drop, allowing the float
needles to come off their seats.
Fuel under pressure from the fuel
pump forces fuel through the inlet
filter past the float needle into
the float bowl, until the desired
fuel level is reached and the float
needles are forced against the
needle seats stopping fuel flow.
As e x p l a i n e d previously, the
floats are spring loaded by a
torsion spring located on the
hinge pin between the float hanger
posts. The purpose of the torsion
spring is to supply additional
pressure on the float arm to as-
sist the float in closing the float
needle valve.
Both primary and secondary sides
of the carburetor float bowl are
individually and internally vented
by vent tubes which transmit the
air pressure from beneath the air
cleaner to the fuel in the float
bowL The air vent tubes balance
the air pressure from beneath the
air cleaner to the fuel in the float
bowl. The amount of fuel needed
by the carburetor is dependent
upon the air pressure in the float
bowl causing fuel to flow. By
locating the vents below the air
cleaner, or internally, the carbu-
retor automatically compensates
for air cleaner restriction, hence
the same pressure causing air to
flow will also be causing fuel to
flow.
A cored passage in the float bowl
slightly above the normal fuel
level, connects the fuel chambers
on the primary and secondary
sides of the float bowl. In this
way, any abnormal rise in fuel
level in one side of the carburetor
bowl will automatically balance
with the other side. It should be
noted the secondary side of the
balance channel extends around
to the rear of the float bowl. This
helps prevent fuel from transfer-
ring from the secondary to the
primary side of the carburetor
when the vehicle is parked down-
hill on a very steep grade.
c. Operation of Idle
(Low Speed) Systems
Each barrel of the carburetor
has a separate idle system but
the general operation is identical
in all barrels. The idle system
in each barrel supplies fuel to
the engine whenever the position
of the throttle valve is such that
suction is created at the idle
discharge holes in the throttle
body.
Suction on an idle discharge hole
causes fuel in the float chamber
to flow through the main metering
jet and upward into the idle tube
which meters the fuel. Calibrated
bleed holes permit air to enter
at the top and side of the idle
passage in the cluster so that a
mixture of fuel and air passes
down the idle channel to the idle
discharge holes. Additional air
is drawn into the fuel-air mixture
in the idle channel through lower
idle air bleeds which are in the
primary side of the main body.
See Figure 3-42.
When the throttle valve is closed,
the fuel-air mixture is supplied
through the lower idle discharge
holes only, since the upper holes
are above the valve and are not
a f f e c t e d by suction. As the
throttle valve is opened, suction
is also placed on the upper idle
discharge holes which then feed
additional fuel-air mixture into
the engine. With continued open-
ing of the throttle valve the suc-
tion on the idle discharge holes
tapers off until a point is reached
where the idle system no longer
supplies fuel-air mixture. Before
this point is reached however, the
main metering system has begun
to supply fuel, as described later.
The lower idle air bleeds dis-
charge fuel after the idle systems
cease to operate, thereby keeping
fuel immediately available in the
idle channels at a point very near
the idle discharge holes and also
enriching the mixture being de-
livered by the main metering
system.
In the primary section, the quan-
tity of fuel-air mixture supplied
through the lower idle discharge
holes is controlled by the idle
needles, which may be adjusted
to provide smooth engine i d l e
operation. In the secondary sec-
tion, the quantity of idle fuel-air
mixture is controlled by the fixed
size of discharge holes located in