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CHOKE SYSTEM (Figure 21)


The purpose of the choke system is to provide an extremely rich fuel mixture for cold engine starting and operation. This rich mixture is required because fuel vapor has a tendency to condense into droplets of raw gasoline upon coming into contact with the cold wall of the intake manifold. This action seriously robs the amount of fuel which normally would reach the engine cylinders to be ignited and burned.

You undoubtedly have noticed that if you place a glass of water in a room how moisture will immediately begin to form on the outside of the glass. This is condensation being formed by the warm air coming into contact with the cool surface of the glass causing beads or droplets of water to be condensed from the moisture in the air collecting on the glass. This same process takes place inside a cold engine.


Whenever a cold engine is cranked with the choke valve closed, the vacuum which is created inside the intake manifold by the downward movement of the pistons will be transmitted inside the carburetor to the underside of the choke valve. By having this low pressure area confined within the entire bore of the carburetor, atmospheric pressure in the fuel bowl will force fuel to flow from both the high-speed nozzles as veil as the idle discharge holes. This action furnishes the necessary rich mixture.

Choke valves may be either manually or automatically operated. In the Rochester 2-barrel line, the model 2G used mostly on trucks and marine engines has a manually operated choke. Models 2GC and 2GV, used mostly on passenger cars, have an automatically operated choke.

Model 2GC means a thermostatic coil and housing assembly operated by hot air will be mounted on the carburetors at either the air horn or throttle body sections. On units with the housing at­tached to the throttle body (some models of Buick, Oldsmobile and Pontiac), an intermediate choke rod adjustment is necessary. Princi­ples of operation are the same regardless of housing location. Full adjustment instructions and procedures are included in Chapter IV as well as the individual instruction sheet supplied in every Echlin carburetor tune-up kit.

Model 2GV means the thermostatic housing is not an integral part of the carburetor assembly. Instead the thermostatic coil is located in a well in the exhaust manifold and is connected to the choke valve by means of a rod. Most automatic choke carburetors today, regardless of the manufacturer, will be of either type listed above.

While the following description of operation pertains to the two types of Rochester choke systems, the general theory will apply to all carburetors in use today.


This choke system consists of a thermostatic coil assembly, off-set choke valve, choke piston, fast idle cam and linkage. The thermos coil is calibrated to hold the choke valve closed when the engine is cold. Because the choke valve is mounted off-center, as the engine starts air velocity against the larger off-set portion causes it to open slightly against the torque of the thermostatic coil spring. Also, intake manifold vacuum which is acting on the choke piston through a vacuum passage tends to open the choke valve.

After the initial start, the choke valve assumes a position where the torque of the thermostatic coil spring is bal­anced against both the vacuum pull on the choke piston and air velocity against the off-set choke valve. As the engine warms up, hot air heated by the exhaust manifold is pulled into the ther­mostatic coil housing. The coil spring, when heated, gradually relaxes its tension allowing the vacuum pull on the choke piston and the air velocity through the carburetor to fully open the choke valve.

Choke Piston Action:

The choke piston tends to modify choke valve action compensating for varying engine loads upon accelerating a cool engine. Upon acceleration or increased engine load, manifold vacuum decreases lessening the pull upon the choke piston. This permits the thermostatic coil spring to momentarily increase choke valve closure providing the engine with a slightly richer mixture for acceleration.

Fast Idle Cam Action:

As a means of preventing engine stalling during the warm-up period, the carburetor linkage is designed to increase engine speed when­ever the choke valve is in a partially closed position. This is accomplished by use of a fast idle cam which is linked to the choke valve shaft. The cam, which has steps of varying height, contacts a screw attached to the throttle shaft. The rotation of the fast idle cam in relation to choke valve opening holds the throttle valves open sufficiently during warm up to increase the idle speed. The greater the choke valve closure, the faster the engine will idle. As the choke valve moves to wide-open position, the idle speed then returns to normal setting.

Choke Unloader Action:

During the warm-up period, the vehicle operator may wish to advance the throttle to wide-open position. Because manifold vacuum now decreases, this would decrease the pull upon the choke piston thereby closing the choke valve, overchoking the engine. As a means of opening the choke valve mechanically to provide for increased carburetor air flow, the throttle lever is made to con­tact a tang on the fast idle cam.  Revolving the fast idle cam results in the positive opening of the choke valve. This choke unloading action also serves to de-choke a flooded engine during starting, whenever the engine is cranked with the accelerator held fully depressed.

Rochester Model 2GV Choke System: (Figure 23)

This type choke system also uses the off-set choke valve, fast idle cam and linkage but will have the thermostatic coil located on the exhaust manifold and connected to the choke valve by a rod. This location permits the thermostatic coil to sense engine heat from the exhaust manifold, thus controlling choke valve position - in the same manner - as air heated by the exhaust manifold and brought to the thermostatic housing, in the 2GC system, permits the thermostatic coil to sense engine heat. Because the metal choke piston in the 2GC system is prone to sticking due to carbon accumulation, a vacuum diaphragm unit mounted on the carburetor air horn and linked to the choke shaft is used in place of the choke piston.

When the engine is started manifold vacuum acting on the diaphragm opens the choke valve to the point where the engine runs without loading or stalling thus performing the same function as the choke piston. The operation of the fast idle cam and linkage is identical to the 2GC system.

Carburetors of all makes and models having an automatic choke will use either the hot air system (2GC type) or the heat- sensing manifold unit system (20. type). Theory of operation will be the same as previously described.

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Table of Contents


Basic Carburetor Theory and Terms

Vacuum and Atmospheric Pressure

Principle of the Four Stroke Engine

Venturi Principle - Principles of Carburetor Operation

Theory of Operation

Main and Power Circuits

Accelerator Pump System

Choke Circuit