Carburetor: Description and Operation

The basic function of the fuel control system is to control fuel delivery to the engine. The L4 2.3L engine uses a Stromberg 2-barrel, 2-stage carburetor.

There are six circuits:
1. Float chamber circuit
2. Slow speed circuit
3. Main circuit
4. Acceleration circuit
5. Step circuit
6. Choke circuit





FLOAT CHAMBER
The float chamber maintains the fuel delivered by the fuel pump at a constant level.





SLOW SPEED CIRCUIT
During idling and at slow speed operation, the throttle valve is almost completely closed. A minimum amount of air is flowing through the venturi. Negative pressure is very low. The main nozzle tip is above the fuel level in the float chamber. The main nozzle is unable to draw fuel, resulting in greater fuel economy at low operating speeds.





MAIN CIRCUIT
The main circuit is most often used during normal vehicle operation.
Opening the throttle valve beyond the specified angle increases the speed and volume of the air passing through the venturi. This results in a greater negative pressure.
The greater negative pressure draws fuel from the float chamber through the main air bleed into the emulsion tube. The emulsion tube mixes the fuel and air.
The air-fuel mixture then passes into the venturi through the main nozzle.





ACCELERATION CIRCUIT
During rapid acceleration, the throttle valve quickly opens completely to draw in a large volume of air. Because there is insufficient fuel in the air horn, the resulting air-fuel mixture will be too lean. A flat spot will occur.
To prevent this flat spot, the accelerator pump forces fuel into the air horn to produce a richer air-fuel mixture.





STEP CIRCUIT
When the carburetor secondary side begins operation, there is an insufficient amount of air passing through the secondary side venturi. The resulting vacuum will not be strong enough to draw the required fuel from the secondary main nozzle. A momentary power loss will occur.
The step circuit acts to supplement the fuel mixture and maintain smooth engine performance.





CHOKE CIRCUIT
The auto choke consists of the eccentric choke valve, the spiral bimetal, the choke diaphragm, and the control rods.
The bimetal closes the choke valve.
The bimetal tension is inversely proportional to the surrounding temperatures. When the engine is cold, the bimetal tension forces the choke valve and the choke diaphragm closed. As the engine warms up, the bimetal tension drops and the choke valve opens.
The choke valve maintains the proper balance between the intake manifold negative pressure and the air horn air flow at all times. This assures stable engine operation.