Buick LeSabre 1965 Service Manual page 117

ENGINE FUEL A N D EXHAUST
4-BARREL ROCHESTER 3 - 4 1
IDLE COMPENSATOR
TOP BLEED
IDLE CHANNEL
RESTRICTION
PRIMARY
IDLE TUBE- '
SECONDARY SUPPLEMENTARY
DISCHARGE HOLES 'DLE HOLES
IDLE NEEDLE HOLES
OFF IDLE OPERATION
Figure 3-42—Primary and Secondary Idle Systems
the rear of the secondary throttle
bores.
On the secondary side of the float
bowl, a thermostatic valve allows
additional air to enter the primary
bores under extreme hot idle con-
ditions. This valve, called the
"idle compensator'', is operated
by a bi-metal strip which senses
temperature. See Figure 3-43.
In a prolonged hot idle the bi-
metal strip bends, raising the
valve and uncovering a hole lead-
ing to the underside of the pri-
mary t h r o t t l e valves. The
additional air drawn into the en-
gine in this manner is sufficient
to offset the enrichening effects
of high temperatures and prevent
hot idle stalling. When underhood
temperatures are lowered, the
valve closes and operation returns
to normal. This valve cannot be
repaired, so a defective valve
must be replaced.
d. Operation of Main
Metering Systems
Each barrel of the carburetor has
a separate main metering system;
however, the operation of all sys-
tems is i d e n t i c a l . The main
metering system in each barrel
supplies fuel to the engine when-
ever the position of the throttle
valve is such that the incoming
air stream creates suction on
the main discharge nozzle.
Air entering the barrel through
the air horn passes through the
venturi tubes which increases the
velocity of the air and create a
suction on the main discharge
nozzle. This causes fuel to flow
from the float chamber through
the main metering jet into the
main discharge nozzle. Air is
drawn in through the high speed
bleeder so that a mixture of fuel
and air is discharged from the
main discharge nozzle into the
air stream passing through the
small venturi in the barrel of the
carburetor. See Figure 3-44.
The main metering systems in
the primary section control the
flow of fuel during the interme-
diate or part throttle range of
operation and up to approximately
85 MPH if the car is accelerated
gradually. The secondary throttle
valves remain closed until the
primary valves have opened ap-
proximately 45-55 degrees, after
which they are opened propor-
tionately so that all valves reach
the wide open position at the same
time. While the secondary throttle
valves are closed, the auxiliary
valves located above them are
held closed by the spring tension
on the a u x i l i a r y valve shaft;
therefore, there is not sufficient
air flow through the barrels to
operate the main metering sys-
tem in the secondary section.
When the s e c o n d a r y throttle
valves are open and engine speed
is about 1600 RPM, the resulting
air flow through the secondary
barrels starts to open the auxil-
iary valves because their sup-
porting shaft is located off-center
in the barrels. The a u x i l i a r y
valves will be fully open at ap-
proximately 2800 RPM. When the
auxiliary v a l v e s are open the
main metering systems in the
secondary section also supply fuel
to the engine. See Figure 3-44.
During the period in which the
secondary throttle v a l v e s are
open and air flow is not high
enough in the secondary bores
to open the auxiliary valves, ad-
ditional fuel is needed for the air
which by-passes around the aux-
iliary valves. This additional fuel
is supplied by tubes which extend
from the mixture channel in the
venturi cluster arm to the low
pressure point below the closed
auxiliary valves. The tubes are
AIR CLEANER
M O U N T I N G -
STUD HOLE |
PRIMARY C •
SIDE \
y
P R I M A R Y / ^ ^ S H
THROTTLE VALVE ^ ^
IDLE
\ AIR
H VALVE
\SKET
' S E C O N D A R Y
SIDE
Figure 3-43—Idle Compensator