Additional Carburetor Control

Additional Carburetor Control

To meet complex fuel economy and emission requirements, carburetors require the help of auxiliary controls. The following describes some of the more common assist devices you are likely to encounter when servicing a carburetor.

Choke Qualifier

Once the engine has started, a leaner mixture is needed. If the choke stays shut, the rich mixture floods the engine and causes stalling. Therefore, the automatic choke has a choke-qualifying mechanism to open the choke plate slightly after the engine has started.

Many integral chokes have a vacuum piston in the choke housing that opens the choke slightly when manifold vacuum reaches a certain level (immediately after the engine starts). Some integral chokes and all divorced chokes have a qualifying diaphragm (also called a choke pull-off diaphragm or vacuum break) instead of a vacuum piston. The diaphragm is connected to manifold vacuum. When the engine starts, the diaphragm retracts, pulling the choke open. The amount of opening, or the distance between the upper edge of the choke plate and the side of the air horn, is called the qualifying dimension or setting. In some carburetors, the pull-off diaphragm has a modulator spring that varies the qualifying setting based on ambient temperatures.


The dashpot is used during rapid deceleration to retard the closing of the throttle. This allows a smooth transition from the main metering system to the idle system and prevents stalling due to the overly rich air/fuel mixture. It also controls the level of HC in the exhaust during deceleration.

The dashpot consists of a small chamber with a spring-loaded diaphragm and a plunger. A link from the throttle comes in contact with the dashpot plunger as the throttle closes. As the throttle linkage exerts force on the plunger, air slowly bleeds out of the diaphragm chamber through a small hole. This slows the closing action of the throttle plate.

Hot-Idle Compensator (HIC) Valve

When the engine is overheated, a hot-idle compensator opens an air passage to lean the mixture slightly. This increases idle speed to help cool the engine (by increased coolant flow) and to prevent excess fuel vaporization within the carburetor. The hot-idle compensator is a bimetal, thermostatically controlled air bleed valve.

Dual Vacuum Break

Some carburetors are equipped with a fuel vacuum break system, which includes a primary diaphragm and a secondary diaphragm. The primary vacuum diaphragm opens the choke valve slightly as soon as the engine starts to keep the engine from overchoking and stalling. The secondary vacuum diaphragm, which is also attached to the choke lever, opens the choke valve slightly wider in warm weather or when a warm engine is being started.

Vacuum to the secondary diaphragm is controlled by a thermal vacuum switch or valve (TVV). The TVV releases vacuum to the secondary vacuum break when the engine reaches a certain temperature. This prevents a rich fuel mixture and the high emissions that result form starting a cold engine in warm weather or when a warm engine is started.

Choke Unloader

To be able to start a cold engine that has been flooded with gasoline, a choke unloader is required. The choke unloader is throttle linkage actuated and opens the choke whenever the accelerator pedal is floored. At wide-open throttle, the partially opened choke allows additional air to lean out the mixture and reduce fuel flow.

Deceleration Valve

The deceleration valve is designed to prevent backfire during deceleration as the fuel mixture becomes richer. The valve, which operates only during deceleration, is usually located between the intake manifold and the air cleaner. A typical valve has a cam-shaped diaphragm housing on one end. A control manifold-vacuum line is attached to a port under the diaphragm housing. The other end of the valve is connected by hoses to the intake manifold and air cleaner. When deceleration causes an increase in manifold vacuum, the diaphragm opens the deceleration valve and allows air to pass from the air cleaner into the intake manifold, leaning the fuel mixture and preventing exhaust system backfire.

Throttle Position Solenoid

The throttle position solenoid is used to control the position of the throttle plate. It can have several function, depending on its application. When the basic function is to prevent dieseling, the solenoid is called a throttle stop solenoid or an idle stop solenoid. When the engine is started, the solenoid is energized and the plunger extends, pushing against the throttle linkage. This forces the throttle plates open slightly to the curb idle position. When the ignition switch is turned off, the solenoid is deenergized and the plunger retracts. This allows the throttle plate to close completely, and it shuts off the throttle plate to close completely, and it shuts off the air/fuel supply to prevent dieseling or run-on.

The throttle position solenoid is also used to increase the curb idle speed to compensate for extra loads on the engine. When this is its primary function, the solenoid might also be called an idle speedup solenoid or a throttle kicker. This feature is most often used on cars with air conditioners. When the air conditioning is turned on, a relay energizes the solenoid so the plunger extends farther, raising the idle rpm. This keeps the engine running at a higher speed, which is required to maintain a smooth idle speed and to ensure adequate emission control.

The throttle position solenoid is also used to control idle speed when the transmission is engaged. A relay in the park/neutral switch signals the solenoid to extend when the transmission is shifted into gear. This opens the throttle slightly to compensate for the increased load on the engine.

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