Buick LeSabre 1965 Service Manual page 115

ENGINE FUEL AND EXHAUST
4-BARREL ROCHESTER 3 - 3 9
Paragraph
3-18
3-19
SECTION 3-F
ROCHESTER 4-BARREL CARBURETOR
Subject
CONTENTS OF SECTION 3-F
Page Paragraph
Subject Page
Description and Operation
of Rochester 4-Barrel
Carburetor 3-39
Disassembly, Cleaning, and
Inspection of Rochester
4-Barrel Carburetor 3-44
3-20
3-21
Assembly and Internal
Adjustment of Rochester
4-Barrel Carburetor
External Adjustment of
Rochester 4-Barrel
Carburetor
3-45
. . . 3-47
3 - 1 8 DESCRIPTION AND
OPERATION OF
ROCHESTER
4-BARREL
CARBURETOR
a. General Description
The Rochester Model 4GC is a
4-barrel down-draft type which
provides the advantages of a com-
pound installation of two 2-barrel
carburetors in one compact unit.
See Figure 3-40. To aid descrip-
tion and the proper identification
of parts the carburetor is con-
sidered to be divided into a
primary section and a secondary
section.
The primary section covers the
2-barrelled forward half of the
carburetor assembly. This sec-
tion is essentially a complete 2-
barrel carburetor containing a
float system, idle system with
adjustable needle valves, main
metering system, power system,
and accelerating system. This
section also includes the auto-
matic choke mechanism.
The secondary section covers the
2-barrelled rearward half of the
carburetor assembly. This sec-
tion is essentially a supplemen-
tary 2-barrel carburetor which
cuts in to assist the primary
section when a predetermined
throttle opening and engine RPM
are reached. This section con-
tains a float s y s t e m , a non-
adjustable idle system, and a
main metering system. It has a
separate set of throttle valves
and a set of auxiliary valves,
which are located in the barrels
above the throttle valves.
The primary throttle valves are
operated by the accelerator pedal
and the connecting throttle link-
age. The s e c o n d a r y throttle
valves are operated by the pri-
mary throttle valve shaft through
delayed action linkage which per-
mits a predetermined opening
of the primary valves before the
secondary valves start to open.
Action of the linkage then causes
both sets of throttle valves to
reach the wide open position at
the same time.
b. Operation of Float System
The model 4GC carburetor uses
two sets of twin floats to maintain
1/
IDLE
MIXTURE
NEEDLES
I / /
THROTTLE
STOP
SCREW
wM
4
TRANSMISSION
SWITCH
PIN
Figure 3-40—Rochester 4-Barrel
Carburetor
correct fuel level in both primary
and secondary sides of the float
bowl u n d e r a l l conditions of
operation.
The primary and secondary float
assemblies are new in that the
pontoons are made of a closed
cell plastic compound. The new
material has less weight and is
consequently more buoyant than
the conventional brass pontoon
and are, therefore, smaller. The
smaller pontoons allow a greater
fuel reserve in the float bowl.
The new float assemblies use
torsion springs wrapped around
the hinge pin which give additional
assist to the float in closing
the float n e e d l e valve which
consequently g i v e s a f a s t e r
reaction f l o a t . The primary and
secondary float assemblies and
torsion springs are completely
interchangeable.
Fuel from the engine fuel pump
enters the carburetor through the
inlet fitting on the primary side
of the air horn. It first passes
through a sintered bronze fuel
filter located just behind the fuel
inlet nut. It should be noted that
the fuel filter is spring loaded.
This provides a pressure relief
feature so that in the event the
filter should plug, restriction will
cause fuel pump pressure to over-
come the spring and allow fuel to
by-pass the filter. After the fuel
passes through the inlet filter it
goes to the primary needle seat
and to the secondary needle seat
through a cross channel cast in
top of the air horn assembly.