The carburetor used on these vehicles is a 2-barrel down-draft fixed venturi type. There are two throttle bores (primary and secondary) in the carburetor. During normal engine operation, fuel is supplied through the primary bore; at high engine speed, fuel is supplied through both the primary and secondary throttle bores.
The carburetor consists of three major components assembled in layers: the air horn, mixing body and throttle body. The choke valve is in the air horn. The venturi, float chamber, accelerator pump, actuator and choke housing are in the mixing body layer, while the primary and secondary throttle valves, secondary diaphragm and pilot screw are part of the throttle body.
See Figure 1
This system provides easy engine starting under a wide range of air temperatures. The system consists of the following:
The choke valve is located in the primary throttle bore of the carburetor. When the engine is not running, the choke valve angle is determined by the bimetallic coil spring acting against the choke return spring. When the engine is running, the choke opener also affects the choke valve angle.
When the engine is started, electric current supplied to the main choke heater causes the bimetallic coil spring to gradually open the choke valve. As the air temperature in the choke cap rises, the thermal switch turns on and electric current is also directed to the secondary choke heater. This speeds the opening of the choke valve during its final stages. The combination of heater and thermistor keeps the bimetallic coil spring at a constant high temperature.
The choke opener adjusts the choke valve for increased air flow once the engine begins to fire. It operates in two steps according to coolant temperature and it operates independently of the fast idle setting.
The fast idle unloader operates in two steps according to the characteristics of two thermovalves, which sensor the engine coolant temperature differently.
CHOKE OPENER SYSTEM
See Figure 2
The fast idle cam is engaged and disengaged by depressing the accelerator pedal and is also disengaged by the fast idle unloading mechanism.
The unloading mechanism consists of a fast idle unloader and thermovalves A and B . The unloader has two diaphragms to release the throttle valve in two steps. When the coolant temperature reaches the first thermovalve temperature setting, the valve closes to shut off the vacuum bleed. This allows the inside diaphragm of the unloader to retract to the first step by manifold vacuum. Then, as the coolant temperature further rises and reaches the second thermovalve temperature setting, the second valve closes and manifold vacuum is applied onto the outside diaphragm of the unloader. This allows the unloader to operate on the second step.
This system is designed to promote easy starting. During engine start-up, manifold vacuum is transmitted to the choke opener; thus, the choke valve is opened a fixed amount.
The first thermovalve works to open the choke valve in response to engine coolant temperature. When the engine coolant temperature is below the set temperature of the thermovalve, it opens and manifold vacuum is bled from the valve. In this situation the choke opener diaphragm is retracted to an intermediate position because of the balance between the vacuum and the spring force of the choke opener.
When the engine coolant temperature exceeds the set temperature of the thermovalve, it closes to shut off the vacuum bleed and this allows the choke opener to fully retract and pull the choke valve open.
See Figure 3
The accelerator pump supplies the extra fuel necessary to maintain the best possible air/fuel mixture when the accelerator pedal is depressed suddenly at low engine speeds.
When the accelerator pedal is depressed, the pump rod, which is connected to the throttle lever, pushes down on the accelerator pump diaphragm. This opens the outlet check valve and allows fuel to be pumped up to the accelerator pump nozzle, where it is sprayed into the carburetor's primary throat.
The fuel flow from the accelerator pump is further regulated by the temperature-sensitive bypass valve in the carburetor. When the engine is cold, the bypass portion of the valve is closed so that the accelerator pump nozzle will get the maximum of fuel available. When the engine warms up to its normal operating temperature, the bypass valve opens and this allows some of the fuel, which would ordinarily be routed to the nozzle, to be bled back into the carburetor float chamber.
CARBURETOR HEAT RISER
See Figure 4
A heat riser, utilizing engine coolant, is built into the intake manifold to prevent carburetor icing and to improve cold driveability. This improves air/fuel mixture atomization. Since excessive heat can cause fuel percolation, a bypass valve with a thermostat shuts off coolant flow at normal temperatures.
VACUUM CONTROLLED SECONDARY
See Figure 5
The secondary throttle valve is controlled by venturi vacuum and thermovalve D. When thermovalve D is below its specified temperature, vacuum to the secondary diaphragm is bled to the air cleaner. Above that temperature, thermovalve D gradually closes the air bleed and supplies venturi vacuum to the secondary diaphragm.
See Figure 6
This system provides supplementary fuel to the primary main fuel passage when the vehicle is run at high load or in the power mode.
When the intake air temperature is below 60°F (16°C), the power valve is open because the power valve control solenoid does not allow vacuum to the power valve.
When the vehicle is below 10 mph, with the intake air temperature above 60°F (16°C), the power valve is still open because the power valve solenoid will not allow vacuum to the power valve.
When the vehicle speed exceeds 10 mph (16 km), the power valve is then controlled by the signal from the vacuum switch B to the power valve control solenoid valve. The power valve control solenoid valve is energized by low manifold vacuum to the vacuum switch.1.5L Engine
The control unit energizes the power valve control solenoid to cut off manifold vacuum to the power valve which supplies fuel to the primary main fuel passage.