Chevrolet Light Duty Truck 1973 Service Manual page 426

6 M - 2 E N G IN E FUEL
MODEL 2GV (2 bbl.) SAE 1-1/4"
CARBURETOR
The Rochester Model 2GV 1-1/4 (small bore 2 bbl.)
carburetor includes some of the following features.
1. A plastic float is the same as is used in the Model
2GV large bore (1 1/2") 2 barrel. Along with the
plastic float the float needle and seat assembly is
longer which improves fuel level control in the float
bowl.
A float bowl dam has been added to the float bowl
adjacent to the pump well area. The purpose of the
float bowl dam is to prevent fuel slosh and help
maintain improved fuel supply to the main metering
jets under all types of vehicle operation.
2. The pump system on the Model 2GV 1-1/4"
carburetor incorporates an expander spring beneath
the pump plunger cup to maintain constant pump
cup to pump well contact.
In addition to the garter spring, a fuel inlet channel in
the bottom of the pump well maintains sufficient fuel
supply to the pump circuit at all times. A small
aluminum inlet check ball seats in the bottom of the well
so that as the pump plunger moves downward, the fuel
will be forced out through the pump jets.
3. The hot idle compensator will not be used on the
Model 2GV 1-1/4" carburetors. Leaner fuel
mixtures and changes in idle speed have made the
compensator unnecessary.
4. The fast idle cam steps have improved fast idle
speed control during the warm-up period.
5. An exhaust gas recirculation system (E.G.R.) is used
on all applications, to control oxides of nitrogen.
The exhaust gas recirculation valve is operated by a
vacuum signal taken from the carburetor. A vacuum
supply tube, installed in the float bowl beneath the spark
tube, connects by a channel to two timed vacuum ports
located just above the throttle valve in the throttle body
bore.
As the throttle valve is opened beyond the idle position,
the first E.G.R. port is exposed to manifold vacuum to
supply a signal to the E.G.R. valve located on the intake
manifold. The second port in the throttle body is located
mid-way between the top of the throttle valve and the
throttle body casting and is positioned higher to act as
an air bleed for the lower port, thereby, modulating the
amount of vacuum signal supplied by it.
As the throttle valve is opened further in the part throttle
range, at higher air flows, the vacuum signal decreases at
the lower port. The upper port then ceases to function as
an air bleed and is gradually exposed to manifold
vacuum to supplement the signal at the lower port. In
this way the E.G.R. valve operation is timed for precise
metering of exhaust gases to the intake manifold, so that
just the right amount of exhaust gases can be added to
the inlet mixtures for proper control of the oxides of
nitrogen.
The exhaust gas recirculation system does not operate
during engine idle.
MODEL 2GV (2 bbl.) SAE 1-1/2
CARBURETOR
The Model 2G, 2GV (large bore 1-1/2") carburetors include
the features:
1. The carburetors are calibrated to meet current
engine requirements.
2. The internal float bowl vent hole located in the air
horn is enlarged for improved float bowl venting
and vapor handling. As there are no external vents,
the carburetor is completely internally balanced.
3. Plastic main well inserts in the main fuel wells
provide improved fuel metering for the main
metering system. The use of the plastic main well
inserts provide improved fuel control in the off-idle,
transfer and part throttle ranges of operation.
The plastic main well inserts surround the main fuel
discharge nozzles and are removable for carburetor
disassembly and cleaning purposes.
4. An exhaust gas recirculation system (E.G.R.) is used
on all applications to control oxides of nitrogen
emissions. The vacuum supply necessary to operate
the exhaust gas recirculation valve is obtained
through timed vacuum ports in the throttle body
which connects through a channel to a tube which is
located beneath the spark tube at the side of the
float bowl.
As the throttle valve is opened beyond the idle position,
the first E.G.R. port is exposed to manifold vacuum, to
supply a signal to the E.G.R. valve located on the engine
manifold. The second port in the throttle body is located
mid-way between the top of the valve and the throttle
body casting and is positioned higher to act as an air
bleed for the lower port, thereby, modulating the amount
of vacuum signal supplied by it.
As the throttle valve is opened further in the part throttle
range, at higher air flows, the vacuum signal decreases at
the lower port. The upper port then ceases to function as
an air bleed and is gradually exposed to manifold
vacuum to supplement the signal at the lower port. In
this way, the E.G.R. valve operation is timed for precise
metering of exhaust gases to the intake manifold, so that
the right amount can be added to the inlet mixtures, for
proper control of the oxides of nitrogen.
The exhaust gas recirculation system does not operate
during the normal curb engine idle.
5. The steps on the fast idle cam have been revised to
improve engine performance during the warm-up
period.
6. A pump fill trough in the pump system is located
just beneath the venturi cluster gasket. The fill
trough is only used on passenger car models and
gives added capacity to the pump system above the
pump discharge ball for improved operation of the
pump system.
LIGHT DUTY TRUCK SERVICE MANUAL