All About Carburetors

All About Carburetors


Air is drawn into the engine by the intake stroke. As vacuum is created, it draws air through the carburetor and venturi into the engine.

A venturi is a streamlined restriction that partly closes the carburetor bore. Air is forced to speed up as it enters the venturi to pass through the restriction. This restriction causes the formation of a vacuum below the venturi. As engine speed increases during acceleration, more air is drawn into the carburetor. As a result, venturi vacuum increases because the greater the velocity of air passing through the venturi, the greater the vacuum.

Venturi vacuum is used to draw in the correct amount of fuel through a discharge tube. As the air flows through the venturi, vacuum draws the fuel from the carburetor bowl into the stream of air going into the engine. More fuel is drawn in as venturi vacuum increases, and less as the vacuum decreases.


The three general stages involved in carburetion are metering, atomization, and vaporization.


Metering is another term for measuring. In the process of carburetion, fuel is metered into the air passing through the barrel of the carburetor. The mixture of air and fuel is called an emulsion. The ideal air/fuel ration at which all the fuel blends with all the oxygen in the air is called the stoichiometric ratio. This ratio is about 14.7:1. If there is more fuel in the mixture, it is called a rich mix. If there is less fuel, it is called a lean mix. The amount of fuel metered into the air is varied in relation to the amount of air passing through the carburetor. Additional factors that influence the amount of fuel metered into the air include engine temperature, load and speed requirements, and the amount of oxygen in the exhaust system.


Atomization is the stage in which the metered fuel is drawn into the airstream in the form of tiny droplets. The droplets of fuel are drawn out of passages called discharge ports.


The surface area of an atomized droplet is in contact with a relatively large amount of surrounding air. In addition, the venturi is a low-pressure area. These factors combine to create a fine mist of fuel below the venturi in the bore. This is called vaporization - the last stage of carburetion. It occurs below the venturi, in the intake manifold, and within the cylinder. Swirl, turbulence, and heat within the intake manifold and cylinder also enhance vaporization.


The throttle plate controls the amount of air and fuel that flows through the carburetor into the engine. It is a circular disc that is placed directly in the flow of air and fuel, below the venturi. It is connected to the driver's throttle pedal so it opens to a vertical position as the pedal is depressed. When the throttle plate is all the way open, there is very little restriction of air. This is a maximum speed condition. As the driver's foot is removed, a spring closes the throttle plate. This restricts the amount of air going into the engine. This is a low speed condition.


Variations in engine speed and load demand different amounts of air and fuel (often in differing proportions) for best performance and present complex problems for the carburetor. At engine idle speeds, for example, there is insufficient air velocity to cause fuel to be drawn from the discharge nozzle and into the airstream. Also, with a sudden change in engine speed, such as rapid acceleration, the venturi effect (pressure differential) is momentarily lost. Therefore, the carburetor must have special circuits or systems to cope with these situations. There are seven basic circuits used on a typical carburetor.

1. Float

2. Idle

3. Off-idle

4. Main metering

5. Full power (or power enrichment)

6. Accelerator pump

7. Choke

Float Circuit

A float circuit or fuel inlet system of a typical carburetor consists of the fuel bowl, fuel inlet fitting, fuel inlet needle valve and seat, and the float. A full screen or filter is usually installed at the fuel inlet to prevent dirty fuel from mixing in the carburetor and causing a problem.

The float system stores fuel and holds it at a precise level as a starting point for uniform fuel flow. Fuel enters the carburetor through the inlet line and passes through an inlet filter to the inlet needle valve and seat. The incoming fuel is captured and stored in the reservoir or fuel bowl. The fuel bowl is normally an integral part of the main casting but can be a separate casting attached to the carburetor body with screws. Carburetors with primary and secondary venturis might have two separate fuel bowls.

The level of the fuel in the bowl is maintained at a specified height by the raising and falling of the float in the fuel bowl. Early floats were made of brass stampings soldered into an airtight lung. Floats made of nitrile rubber, a closed cell material made of thousands of tiny hollow spheres, are used most exclusively on domestic cars. Hollow plastic floats can also be found in carburetors.

As fuel enters the bowl, the float, which is connected to a hinged lever, rises and closes the inlet needle valve. With the needle valve closed, fuel is prevented from entering the carburetor. Fuel pressure against the inlet needle valve tends to force it open while the buoyancy of the float in the bowl tends to force it closed. This action establishes the precise fuel level for the carburetor. To prevent the float from bouncing and vibrating, a bumper spring is usually installed under the float or to a tang connected to the float.

The metering systems of a carburetor are designed to function properly only when the fuel level in the bowl is at a specific level . The specific level is adjusted with  carburetor partially disassembled on most models. However, it can be adjusted externally on a few carburetors by turning a threaded inlet valve assembly.

The fuel bowl is vented internally to the air horn by a vent tube in the carburetor body. Prior to the introduction of emission controls, most primary fuel bowls were vented to the atmosphere when the engine was at idle or turned off. Since the introduction of evaporative control systems, all fuel bowls are vented by a valve to a charcoal canister, which absorbs and stores fuel vapors. The vapors are returned to the engine when it is restarted.

Idle and Off-idle Circuits

At idle, the engine requires a richer air/fuel mixture than during normal cruising conditions. This is because residual exhaust gases remain in the combustion chambers during low-engine rpm and dilute the air/fuel charge. The idle circuit supplies the richer air/fuel mixture to operate the engine at idle and low speeds.

During idle conditions, there is not enough air entering the venturi to cause a vacuum to move the fuel. The throttle plate is almost all the way closed. During this condition, there is a large vacuum below the throttle valve. This vacuum causes fuel to be drawn from the carburetor float bowl through internal passages to the idle port below the throttle plate. As fuel is drawn from the float ball to the idle port, air is drawn in through an  air=bleed passageway near the top of the carburetor. Only a small amount of air passes by the throttle plate. The end result is the richer air/fuel mixture needed for idle operation.

As the throttle plate is opened during low-speed operation, a transfer slot located above the throttle plate is progressively exposed to a vacuum, and the air/fuel emulsion is also discharged  from the transfer slot. The increased air/fuel mixture flow provides a smooth transition between idle and cruising modes of operation. Some carburetors have a series of holes called off-idle air passages, instead of a transfer slot. Like the transfer slot, the holes permit increased fuel delivery as the throttle opens. This is called an off-idle system.

On older carburetors there is an idle mixture needle valve. This valve is used to control or adjust the amount of air and fuel at idle. More current carburetors, however, have limiting caps on the idle mixture screws, which limit the amount of adjustment available. On newer carburetors, the idle mixture screws are sealed with steel plugs to eliminate all adjustment.

To improve idle quality when meeting emission standards, a variable air bleed idle system is used on some carburetors. In this system there are two idle air bleeds. One idle air bleed is installed normally in the air horn. An auxiliary idle air bleed is drilled into the lower skirt of the venturi. The air entering through the auxiliary passage is adjusted by an idle air adjusting screw. The screw is turned clockwise to enrich the idle mixture and counterclockwise to lean out the idle air/fuel mixture.

Main Metering Circuit

The main metering circuit comes into operation as engine speed increases. Opening the throttle plate past the idle position increases the air flowing through the venturi and creates enough vacuum to allow atmospheric pressure to force fuel through the main metering system and out the main duel discharge nozzle, located in the center of the venturi. As engine speed is increased, the vacuum at the discharge nozzle increases.

This vacuum or pressure differential causes fuel to flow out of the fuel bowl, through the main metering jet, and into the main well. On most carburetors, the main well is vented through a precisely sized opening called the main well air bleed. The main well air bleed allows air to enter at the top of the main well. Air entering the calibrated main air bleed prevents a vacuum from developing in the main well. The air also allows for aeration of the fuel as it leaves the main well and travels up the well tube. This allows the fuel to be partially atomized as it travels toward the discharge nozzle.

Air flows through the bleeds because it draws air from the high-pressure area above the venturi. The main metering discharge nozzle is in the low-pressure venturi level.

As the air speed increases through the venturi, more fuel is drawn from the main well. This lowers the fuel level in the main well and exposes more air bleed openings in the main well tube. This causes extra air to enter the well tube, mix with the fuel, and dilute or lean the mixture. This action circumvents the richening effect caused by the increased carburetor airflow. If the fuel were not diluted as such, the air/fuel mixture would richen at high speed as the venturi vacuum increased faster than the engine's need for additional fuel.

Secondary Metering System Some carburetors have more than one barrel or air horn. Each barrel has a throttle plate and main metering system (as well as other circuits). When all throttle plate open and close simultaneously, the carburetor is called a single-stage carburetor.

Some carburetors have two stages, called primary and secondary stages. In the primary stage, one or two throttle plates operate normally as in a single-stage carburetor. The secondary stage throttle plates, however, only open after the primary throttle plates have opened a certain amount. Thus, the primary stage controls off-idle and low cruising speeds. The secondary stage opens when high cruising speeds or loads require additional air and fuel. The added flow capacity raises the engine power output.

The secondary throttle plates can be opened mechanically or by a vacuum source. Mechanically actuated secondary throttle plates are opened by a tab on the primary throttle linkage. After the throttle primary plates open a set amount (usually 40 to 45 degrees), the tab engages the secondary throttle linkage, forcing the plates open. Vacuum-actuated secondaries have a spring-loaded diaphragm. Vacuum is supplied to the diaphragm from ports in the primary and secondary throttle bores. When the vacuum in the primary bore reaches a specific level, the vacuum supplied to the diaphragm overcomes the spring and opens the secondary throttle plates. The vacuum created in the secondary throttle bore increases the vacuum signal to the diaphragm, opening the secondary throttle valves still farther.

Power Enrichment Circuit

At wide open throttle, the engine needs a richer-than-normal air/fuel mixture. This mixture cannot be supplied by the main metering system. So, an additional fuel enrichment or full-power system is provided on most carburetors. The power enrichment system meters additional fuel into the mixture. This can be accomplished in several ways.

Metering Rods In some carburetors, power enrichment is provided by metering rods placed in the main jets. The metering rods are actuated mechanically or by vacuum. When the throttle is not wide open (or nearly so), the throttle linkage keeps the rods in the jets, providing normal fuel flow. When the throttle is opened wide, either a mechanical link in the throttle linkage or vacuum-actuated lever lifts the rods out of the jets, enabling more fuel to be forced into the main well. The additional fuel flow richens the air/fuel mixture.

Power Valve The power valve is basically a vacuum-operated metering rod. It consists of a vacuum diaphragm or piston, a spring-loaded valve, and a metering rod inside an auxiliary fuel jet usually located in the bottom of the fuel bowl. A vacuum passageway machined into the main body casting supplies manifold vacuum to the diaphragm or piston. During idle and low cruising speeds, the vacuum holds the power valve closed. As engine speed and load increases and the vacuum signal drops to a specific level, the spring overcomes the vacuum and forces the power valve out of the jet. This increases the fuel flowing into the main well.

The power valve has been replaced or modified in today's feedback carburetor. In a feedback system, an electrical solenoid controls the metering fuel jets or idle air bleeds to regulate the air/fuel mixture. Feedback carburetors are discussed later in this chapter.

Accelerator Pump Circuit

The off-idle or transfer circuits discussed earlier allows the engine to be accelerated smoothly without hesitation or lags. However, during sudden acceleration, the engine experiences a momentary drop in power unless additional fuel is simultaneously introduced into the air charge.

During sudden acceleration, the air flowing through the carburetor reacts almost immediately to each change in the throttle plate opening. However, since fuel is heavier than air, it has a slower response time. Fuel in the main metering system or idle system takes a fraction of a second to respond to the throttle opening. This lag in time creates a hesitation of fuel flow whenever the accelerator pedal is quickly depressed. The accelerator pump system solves this problem by mechanically supplying fuel until the other fuel metering systems are able to supply the proper mixture.

One type of accelerator pump is the diaphragm type located in the bottom of the fuel bowl. Locating the pump in the bottom of the fuel bowl ensures a more solid charge of fuel (fewer bubbles).

When the throttle is opened, the pump linkage, activated by a cam on the throttle lever, forces the pump diaphragm up. As the diaphragm moves up, the pressure forces the pump inlet check ball or valve onto its seat. This prevents the fuel from flowing back into the fuel bowl. At the same time, the pressure of the fuel causes the discharge check ball or valve to rise and fuel is then discharged into the venturi.

As the throttle returns toward the closed position, the linkage returns to its original position and the diaphragm return spring forces the diaphragm down. The pump inlet check valve is moved off its seat and the diaphragm chamber is refilled with fuel from the fuel bowl.

Another common type of accelerator pump uses a plunger rather than a diaphragm. As the throttle moves toward the open position, pressure from the plunger forces an inlet ball onto a check valve, sealing the inlet valve. At the same time, a ball is forced off the outlet check valve and fuel is discharged from the shooter nozzle. As the throttle moves back toward the closed position, the plunger retracts, allowing the inlet check valve to open and fuel to refill the pump.

Choke Circuit

A cold engine needs a very rich air/fuel mixture during cranking and startup. Providing the rich mixture is the job of the choke circuit.

During a cold startup, the choke should be closed. This creates a very high vacuum level in the air horn below the choke plate. As the air pressure outside the carburetor forces its way into the low-pressure areas, it draws with it a rich air/fuel mixture. When the throttle plate is closed, the mixture is forced out through the idle port or ports below the throttle valve. If the throttle valve is opened to assist in starting the engine, additional ports are exposed to the low-pressure manifold pressure and additional fuel is forced into the air horn. After the engine starts, a leaner mixture can be used to keep the engine running. Therefore, the choke should be opened some to allow increased airflow. After the engine has warmed to normal operating temperatures, the choke should be opened completely to allow the throttle to control airflow and fuel metering.

Before the introduction of automatic chokes, the opening and closing of the choke plate was manually controlled by the driver. A choke cable was connected to a knob inside the passenger compartment on the dash. To close the choke, the driver simply pulled the knob out. As the engine warmed, the choke knob was gradually pushed in to open the choke.

Modern carbureted vehicles have an automatic choke that operates without any driver assistance. Being more sensitive to engine temperature, an automatic choke is more efficient.

The typical automatic choke has a bimetal coil called a thermostatic spring. When the coil is cold, it forces the choke plate closed. As the bimetal coil warms, it expands and pulls the choke plate open.

The bimetal coil can be mounted directly on the carburetor. This type is called an integral choke. The bimetal coil may also be mounted on the intake manifold or in a heat well in the exhaust heat passage of the intake manifold. This type is called a divorced or remote choke.

The integral choke normally has a heat source to warm the bimetal. The heat source might be hot air or coolant. Many integral chokes also have an electrical heater to assist in warming the coil during warm weather or hot startup. The bimetal coil on many feedback carburetors is heated solely by a solid state heating element. Voltage is usually provided to the heating element from the alternator circuit.

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