carburetor evolution

How do carburetors work?

Carburetor Systems

The complete function of the carburetor can be best described by explaining the various control points and the influencing air and fuel calibrations and passages. These are normally considered to be the following:

  • Float System
  • Idle or Low Speed System
  • Main Metering System
  • Power System
  • Accelerating System
  • Choke System

For the purpose of illustration and simplification, we will use the single barrel carburetor only.

Float System

Fig 17 Float sytem

The float system controls the level and supply of gasoline in the fuel chamber throughout the operating range of the engine. Fuel under pressure is delivered to the fuel inlet. It passes through the float valve seat, around the float valve, into the fuel chamber. Raising the fuel level in the fuel chamber forces the float upward. This shuts off the flow of gasoline when the desired level has been reached by forcing the float valve against its seat. As the fuel flows through the carburetor jets into the engine, the fuel level and the float drop, allowing additional fuel to enter the fuel chamber.

In actual operation, the float assumes a position that will allow the valve to open just far enough to replace the fuel at the same rate at which it flows from the fuel chamber.

Atmospheric pressure is admitted to the fuel chamber by either an external or an internal vent passage.

An external vent is located in the fuel chamber cover and provides a direct entry for atmospheric pressure.

Fig 18 External Fuel Chamber Vent

An internal vent consists of a passage from the fuel chamber to a point in the carburetor throat between the choke and the air cleaner. This assures equal pressure at this point in the carburetor throat and the fuel chamber.

Pressure at these two points must be the same in order to maintain proper pressure difference between the fuel chamber and the venturi. This method of venting compensates for air cleaner restriction, caused by dirt accumulation, which would reduce the pressure of the incoming air. If the pressure in the fuel chamber was not equal to the pressure of the incoming air, the resulting increase in pressure difference between the fuel chamber and the venturi, would cause more fuel to flow and the carburetor to run rich.

Fig 19 Internal Fuel Chamber Vent


The idle or low speed system consists of the fuel metering orifice, air bleeds, adjusting screw and discharge holes. The idle or low speed system is employed to furnish the proper mixture for the engine idle, light load and slow speeds. At closed throttle and slow engine speeds (light load), the fuel is delivered through the idle system.

When the throttle valve is in the idle position (nearly closed), the air must pass through the opening which exists between the throat and the throttle valve. This produces a low pressure area. Sufficient pressure differential is present between the fuel chamber and the idle discharge to push the gasoline from the chamber.

Fuel from the fuel chamber is pushed through a fuel passage and a fuel calibration into the idle passage. Air is admitted to the idle passage through one or more air bleeds. These air bleeds may be specific calibrations or may be variable, with an adjustment. The air which is admitted, mixes with the fuel and aids in its atomization.

Fig 20 Idle or low speed system

The air bleed also prevents siphoning through the idle system when it is not operating.

The discharge holes are located in definite relation to the edge of the throttle valve at closed position.

A portion of the idle discharge opening must be located on the engine manifold side of the closed throttle position. This location provides the low pressure in the idle passage which causes fuel to flow for idle operation.

The idle discharge holes are made in a variety of sizes and shapes. As the throttle valve is opened, it will not only allow more air to flow around it, but will also uncover more of the idle discharge hole. This allows a greater quantity of the gasoline and air mixture to enter the carburetor throat from the idle passage. In some cases there are two or more of these idle discharge holes which are progressively uncovered.

Two methods of adjusting the idle may be employed. In one, the adjustment screw controls the amount of air bled into the system. In the other, the adjusting screw controls the quantity of fuel and air mixture discharged.

Fig 21 Idle adjusting screw controlling air bled into system
Fig 22 Idle adjusting screw controlling quantity of fuel and air mixture

The idle system is overlapped by the main metering system. This overlap is known as the “transfer range”. As the throttle valve is opened to increase speed, the velocity of the air through the venturi increases. The effective pressure differential is then gradually transferred to the main metering system. During the transfer range, fuel is supplied by both the idle and the main metering systems. The supply of fuel by the idle system decreases as the supply through the main metering system increases.


The main metering system consists of a fuel metering orifice which may be variable or fixed, and a passage which leads to the discharge jet or nozzle. An air bleed or well may be incorporated in this passage.

When the air passes through the venturi, its pressure is reduced. Since the pressure in the fuel chamber is greater than the pressure in the venturi, fuel is forced through the main metering system.

Fig 23 Main metering system

The fuel flows from the fuel chamber through the main metering orifice or jet, then through the fuel passage and is then discharged into the air stream through the discharge jet or nozzle.

The amount of air which passes through the venturi is regulated by the throttle control. As the throttle valve is opened to increase speed, a greater volume of air passes through the venturi.

This increase in air volume causes a greater drop in pressure. The resulting increase in pressure difference causes a greater flow of fuel. The increased amount of fuel would exceed engine requirements. An air bleed is therefore incorporated in the system to regulate the rate of flow to prevent the excess. This avoids rich mixtures and wasted fuel.

A metering rod is used in some carburetors to vary the effective size of the main metering orifice. Several steps or tapers are machined on the lower end of the rod and this end is installed in the main metering orifice. As the throttle valve is opened, the rod is raised in the orifice by throttle linkage. The effective size of the orifice is changed as the rod is raised or lowered. The fuel flow is thereby regulated to meet engine requirements.

Fig 24 Metering rod operated mechanically

Metering rods are also controlled by either a diaphragm or piston which operate by pressure difference. A connecting passage leads from one side of the diaphragm, or from the end of the cylinder in which the piston moves, to the carburetor throat, on the engine manifold side of the throttle valve.

Fig 25 Metering rod operated by pressure difference – piston and diaphragm types

When the throttle valve is in the idle position and continuing to a position somewhat above half throttle, the low pressure in the manifold is transmitted through the connecting passage to either the diaphragm or the piston. A pressure difference is established which causes either of, the parts to move toward the side of lower pressure. The difference in pressure is also sufficient to compress the spring which is used on the linkage that connects either of these parts to the metering rod.

When the throttle valve is moved beyond the point indicated above, toward full open position, the pressure in the engine manifold increases. At a predetermined point, the pressure of the compressed spring is sufficient to overcome the pressure difference and the diaphragm or piston moves in the opposite direction. The alternate movement thus moves the metering rod up and down in the metering orifice and regulates the flow of fuel.

The main metering system supplies fuel for the total throttle range from transfer point to full throttle. For full power or top speed, additional fuel is required to maintain proper mixture ratio. This supplemental fuel is supplied by the power system.


The power system consists of a metering orifice with a control mechanism, and a passage into the main metering system. There are several methods which may be used to actuate the control mechanism and provide the additional fuel for maximum power and high speed operation. These methods are: Pressure difference, and mechanical.

A mechanical system consists of an extra arm on the pump rod which contacts and opens the power jet valve when the throttle is opened. Mechanical power systems are not common on current applications.

Fig 26 Power system operated by pressure difference (piston and diaphragm controlled)

Pressure Difference Types

There are two ways of applying the pressure difference principle. One method uses a piston or diaphragm actuated by pressure difference and is normally referred to as “vacuum operated”. The other method is referred to generally as the “back suction” type.

Piston or Diaphragm

The diaphragm or piston is operated by pressure difference. The piston is installed in a cylinder. A compression spring operates the piston in one direction. It moves in the opposite direction because of pressure difference. This pressure difference exists in a passage between one end of the cylinder and the carburetor throat on the intake side of the throttle valve.

When the throttle valve is in idle position, through the part throttle position, sufficient low pressure exists in the manifold and the connecting passage to move the piston to the end of the cylinder where the passage connects. This force also compresses the spring. When the throttle valve is moved toward the full open position, the pressure in the manifold and passage increases. This allows the force of the spring to move the piston in the opposite direction. The alternate movement of the piston causes the power jet to open or close. In this manner fuel is supplied through the power system accordingly to engine load and speed requirements.

When a diaphragm control is used, the same principles and basic method of operation apply. The only difference between the diaphragm and the piston type is the style and shape of the parts involved.

In the case of a metering rod control in the power system, the action is identical. The movement of the metering rod controls the variation in the effective size of the metering orifice.

Back Suction

Carburetors using this system do not employ a separate power metering orifice. Instead, the pressure in the fuel chamber is varied by an air passage into the throat of the carburetor. The application of this principle uses a passage which terminates in the carburetor throat at a point on the venturi side of the throttle valve. The passage originates in the fuel chamber.

This method reduces the pressure in the fuel chamber at part throttle operation by transmitting the lower manifold pressure to the fuel chamber.

Fig 27 Two types of back suction operated power systems

The pressure difference is thereby reduced between the fuel chamber and the venturi. Because of the lesser pressure difference, less fuel will flow at part throttle operation. This lessened fuel flow effects operating economy for light load requirements. For full load or full throttle requirements the change in the throttle valve position increases the pressure difference and the additional fuel requirements are provided.


The accelerating system consists of an inlet check valve, cylinder, pump piston or diaphragm, outlet check valve, and pump or accelerating jet. Some pump pistons or diaphragms are actuated mechanically while others are controlled by pressure difference.

Fig 28a Accelerating system operated mechanically
Fig 28b Accelerating system operated by pressure difference

When the throttle is opened suddenly, air rushes through the carburetor and the intake manifold. Since air is lighter than liquid fuel it gets into motion sooner and would reach the manifold before the fuel supplied by the main metering system. This would result in a momentarily lean mixture. To counteract this condition, additional fuel must be supplied.

The accelerating system controls a small amount of fuel that is momentarily discharged into the air stream when the throttle is opened quickly. This extra amount of fuel is necessary to insure instantaneous response from the engine on acceleration.

Fuel flows from the fuel chamber into the pump cylinder through the inlet check valve as the throttle is closed. When the throttle is opened, it causes the pump piston or diaphragm to exert pressure on the fuel in the pump cylinder. This increase in pressure closes the inlet check valve, opens the outlet check valve and forces fuel through an accelerating passage and calibrated jet into the air stream. In some cases an additional check valve or a vented accelerating jet may be used to prevent siphoning or fuel delivery from the accelerating system at high speeds.

Mechanical System

The piston or diaphragm is moved by throttle linkage in mechanically operated systems. Sudden pressure on the pump piston or diaphragm, created by instant throttle opening, would create tremendous pressure on the fuel in the pump cylinder. A spring is incorporated to avoid bending the linkage but still maintain full pump discharge pressure until the entire pump charge has been forced through the pump jet. This same action can be accomplished by a dry pump using a pocket of air below the plunger for delayed action.

Pressure Difference System

The movement of the piston or diaphragm is controlled by spring tension and pressure difference in pressure differential systems. A passage leads from the carburetor throat on the intake manifold side of the throttle valve to one side of the diaphragm or to one end of the cylinder in which the piston moves. When the throttle valve

is in the idle position and continuing to a position somewhat about half throttle, the low pressure then existing in the manifold is transmitted through the connecting passage to either the diaphragm or piston. A pressure difference is established which causes either of the parts to move toward the side of lower pressure. This difference in pressure is also great enough to compress the pump spring.

When the throttle valve is moved suddenly toward the open position, higher pressure in the manifold is transmitted through the passage and the pump spring then moves the piston or diaphragm to exert pressure on the fuel in the cylinder. This pressure closes the inlet check valve, opens the outlet check valve, and forces fuel through the calibrated jet into the air stream.

Accelerating System Piston Travel Adjustment

On many carburetors provision is made for adjustment which controls the amount of piston travel. This is usually provided in the linkage and consists of selective holes or grooves into which the pump linkage can be positioned to vary piston travel for various operating conditions.


When engine temperatures are low and the manifold is cold, much of the fuel in the air-fuel mixture condenses and remains in liquid form in the manifold. Under these conditions only the light, volatile fractions of the gasoline vaporize. Closing the choke while cranking the engine causes a larger amount of fuel to be delivered to the manifold which in turn results in enough of the light and volatile fractions being made available to allow combustion. As manifold temperature increases, progressively more and more of the fuel is vaporized and less choking is required. The choke system may be operated either manually or automatically.

Manual Choke

A manual choke consists of a choke shaft and valve and operating linkage. A fast idle control is sometimes incorporated in the operating linkage.

Fig 29 Manual choke with linkage and fast idle cam

Choke valves do not close air tight. Some opening is provided to admit enough air to support combustion. As the engine starts and exceeds cranking speed, this in turn will cause a decrease in the manifold pressure and will result in excessive fuel flow. To prevent flooding under these conditions, additional air must be admitted immediately. This is accomplished by means of some type of spring loaded release mechanism which opens automatically as soon as the engine starts.

Fig 30 Spring loaded relief poppet valve

In one design, the choke valve incorporates a spring-loaded poppet valve. The poppet is held

in the closed position by a spring. As soon as the engine starts, pressure differential opens the poppet valve, allowing sufficient air to flow to prevent stalling. In the other design, the choke valve is positioned off-center and is operated by a coiled spring on the end of the choke shaft. In the full choke position, the spring holds the valve closed. As soon as the engine starts, the increased pressure differential overcomes the spring tension and opens the valve part way.

Some choke valves contain only a relief hole to admit air to prevent stalling.

Automatic Choke

An automatic choke usually consists of a choke shaft and valve, a housing, thermostatic coil, choke piston and cylinder and fast idle cam and linkage.

The automatic choke eliminates manual control of the choking operation. It provides the proper amount of choking to enrich the fuel mixture for all conditions of engine operation during the warm-up period.

The automatic choke is controlled by a combination of intake manifold pressure, atmospheric temperature, and exhaust manifold or electric heat.

Fig 31a Automatic choke operated by electric heating element
Fig 31b Automatic choke operated by manifold heat

The bimetallic thermostatic coil is made up of two metals with different coefficients of’ expansion. This coil is attached to the housing on one end and is linked on the other end to the choke valve shaft. The coil is calibrated to hold the choke valve in the closed position when the engine is cold.

Fig 32 Bimetallic choke spring

As the engine is started, difference in pressure above and below the offset choke valve causes the valve to open slightly against the torque of the thermostatic coil. In addition, when the engine starts, intake manifold pressure is applied to the choke piston, which also tends to open the choke valve. The choke valve assumes a position where the torque of the thermostatic coil is balanced against the opposing forces. These forces are: (1) air pressure against the longer side of the offset choke valve, and (2) the force of the choke piston which moves in its cylinder toward the area of low pressure created through the connecting passage. This passage originates on the intake manifold side of the throttle valve in the carburetor throat. The choke action provides a regulated air flow into the carburetor which furnishes a proper mixture during the warm-up period.

The tension of the thermostatic spring is controlled by heat transfer. There are two sources of heat which may be used to control the choke spring. One is through a connecting passage between the exhaust manifold heat chamber (known as a “stove”) and the choke housing. The other source is an electric resistance coil mounted in the choke housing. The heat causes the thermostatic spring to relax its tension and allows the choke valve to open.

Fig 33 Fast idle cam and linkage

To prevent stalling during the warm-up period, it is necessary to run the engine at an idle speed slightly higher than that for a warm engine. This is accomplished by the fast idle cam which is linked to the choke valve shaft and holds the throttle valve open sufficiently during the warmup period to give the increased idle R.P.M. until such time as the choke valve moves to the full open position.


While the automatic choke is in operation, the operator may wish to advance the throttle to the full wide open position. Since this would increase pressure in the manifold, it would relieve the piston force. The spring force, being greater, would then close the choke valve. Some provision must be made to open the choke valve under these conditions.

To accomplish this, there is a projection on the throttle lever. As the throttle lever is moved to the wide open position, the projection contacts the fast idle cam and moves the cam and linkage a short distance. This will open the choke valve sufficiently to prevent stalling.

This device is known as the unloader.

Fig 34 Undloader

The principal function of the unloader mechanism is to provide a mechanical means of partially-opening the choke valve. This is desirable when cranking the engine if it fails to start at once. Keeping the choke fully closed for prolonged cranking periods would tend to “load” the manifold with an excessively rich fuel-air mixture. Such mixture would not burn in the combustion chamber and would make starting difficult. To relieve this condition, a cam and lever are incorporated in the throttle linkage of the carburetor. Fully depressing the accelerator or throttle will cause engagement of the cam and lever and the choke valve will be opened partially. This action will admit more air through the carburetor throat and relieve the rich condition. When the throttle is moved to part throttle or idle position, the cam and lever disengage and the automatic choke functions in its normal manner.


Many carburetors employ accessory devices which are incorporated in or attached to the carburetor to perform functions not basically involved in actual carburetion. A “slow closing throttle’’ mechanism and a “kickdown switch” are examples. These items are generally special as to type, design and application. Description and functional explanation should be obtained from the individual vehicle or carburetor manufacturer.

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