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Webber Sidedraft 45 dcoe carb

The carburetor (American spelling, carburettor or carburetter in Commonwealth countries, "carb" for short, or "carbie" (slang)) is a device which mixes air and fuel for an internal-combustion engine. Carburetors are still found in small engines and in older or specialized automobiles such as those designed for stock car racing. However, most cars built since the early 1980s use computerized electronic fuel injection instead of carburetion. The majority of motorcycles still are carburated due to lower weight and cost, however as of 2005 many new models are now being introduced with fuel injection.

Most carbureted (as opposed to fuel-injected) engines have a single carburetor, though some, primarily with greater than 4 cylinders or higher performance engines, use multiple carburetors. Older engines used updraft carburetors, where the air enters from below the carburetor and exits through the top. This had the advantage of never "flooding" the engine, as any liquid fuel droplets would fall out of the carburetor instead of into the intake manifold; it also lent itself to use of an oil bath air cleaner, where a pool of oil below a mesh element below the carburetor is sucked up into the mesh and the air is drawn through the oil covered mesh; this was an effective system in a time when paper air filters did not exist. Today, most automotive carburetors are either downdraft (flow of air is downwards) or side-draft (flow of air is sideways). In the United States, downdraft carburetors were almost ubiquitous, partly because a downdraft unit is ideal for V engines. In Europe, the side-draft replaced downdraft as underbonnet space decreased and the use of the SU-type carburetor increased. Small propeller-driven flat airplane engines still have the updraft carburetor, as do many small engines, as on lawnmowers and so on.

The carburetor works on Bernoulli's principle: the fact that moving air has lower pressure than still air, and that the faster the movement of the air, the lower the pressure. Generally speaking, the throttle or accelerator does not control the flow of liquid fuel. Instead, it controls the amount of air that enters the carburetor. Faster flows of air and more air entering the carburetor draws more fuel into the carburetor due to the partial vacuum that is created.


Carburetors are either:

  • Fixed Choke (Venturi) - the varying air pressure in the venturi alters the fuel flow; this is the common downdraft carburetor found on American and most Japanese cars
  • Constant depression - the jet is varied by the air flow to alter the fuel flow; the most common, variable choke (constant depression) type carburetor is the sidedraft SU, which was simple in principle to adjust and maintain. This rose to position of domination in the UK car market for that reason. Other, similar designs are used on some European and a few Japanese automobiles.

The carburetor must:

  • Measure the airflow of the engine
  • Deliver the correct amount of fuel to keep the fuel/air mixture in the proper range (adjusting for factors such as temperature)
  • Mix the two finely and evenly

This job would be simple, were air and gasoline both ideal fluids; in practice, however, their deviations from ideal behavior due to viscosity, fluid drag, inertia, etc. require a lot of complexity to compensate at exceptionally high or low engine speeds. A carburetor must provide the proper fuel/air mixture under a wide variety of different circumstances and engine speed range:

  • Cold start
  • Idling or slow-running
  • Acceleration
  • High speed / high power at full throttle
  • Cruising at part throttle (light load)

In addition, modern carburetors are required to do this while maintaining low rates of exhaust emissions.

To function correctly under all these conditions, most carburetors contain a complex set of mechanisms to support several different operating modes, called circuits.


While in fuel injection the jobs of metering airflow, delivering an appropriate volume of fuel, and mixing the two are separated and handled by separate components, in a carburetor this is all done automatically, by the principle of the venturi. A carburetor basically consists of an open pipe, the carburettor's "throat" or "barrel", through which the air passes into the inlet manifold of the engine. The pipe is in the form of a venturi - it narrows in section and then widens again, causing the airflow to increase in speed in the narrowest part. Below the venturi is a butterfly valve or throttle - a rotating disc that can be turned end-on to the airflow, so as to hardly restrict the flow at all, or can be rotated so that it (almost) completely blocks the flow of air. This valve controls the flow of air through the carburetor throat and thus the quantity of air/fuel mixture the system will deliver, thereby regulating engine power and speed. The throttle is connected, usually through a cable or a mechanical linkage of rods and joints or rarely by pneumatic link, to the accelerator pedal on a car or the equivalent control on other vehicles or equipment.

Fuel is introduced into the air stream through small holes at the narrowest part of the venturi. Fuel flow in response to a particular pressure drop in the venturi is adjusted by choosing small brass screws with finely calibrated holes, referred to as jets, into the fuel path. Some original equipment carburetors have very few sizes of jets from which to choose, while aftermarket high performance carburetors typically have a wide range of jets to precisely tune the engine (but in practice, when adding an aftermarket carburetor to an engine, it will usually run quite well without fine tuning the jets if the optimal highest power is not mandatory).

Idle circuit

When the butterfly valve is closed or nearly closed, the carburetor's idle circuit is in operation. The closed throttle reduces the airflow through the venturi to a level which cannot overcome the resistance to flow of the fuel, but it also means that a fairly significant vacuum occurs behind the closed butterfly valve. This manifold vacuum is sufficient to pull fuel through small openings placed after the butterfly valve (and in SU and similar sidedraft carburetors to pull the piston and metering rod up). Only a fairly small amount of air and fuel can pass through in this manner. Since this small volume of fuel/air mixture can generate so little force to keep the engine turning, keeping it running at idle is more difficult than keeping it running at higher speeds. Since the airflow is too low for the carburetor to respond at all, it cannot compensate for fluctuations;instead, idle airflow is set manually by the technician or mechanic, adjusting a screw which opens the throttle a tiny fraction to allow a minimal amount of air to pass, and another screw which serves as a valve in the idle fuel circuit to adjust the volume of fuel delivered. These adjustments interact with each other, as well as affecting manifold vacuum which affects distributor spark advance which in turn affects idle speed, so that adjusting the idle to optimum (highest manifold vacuum at the specified engine idle speed) is not a completely trivial operation. While experts often claim the ability to set the idle perfectly by ear, most individuals do a better job using a tachometer and vacuum gauge. Since the advent of emissions controls on production automobiles, the idle fuel flow is typically set at the factory on the "lean" side of optimal, by restricting fuel flow so that idle speed falls by 100 - 150 rpm from where it was when optimally adjusted, in order to reduce unburned hydrocarbons and carbon monoxide with some slight disimprovement in reliable and smooth idling; the idle fuel adjustment is typically sealed at the factory to prevent tampering, so that adjustment when age and wear cause a large deviation from proper operation requires drilling out a plug over the adjusting screw or some similar modification to gain access.

Off-idle circuit

As the throttle is opened up slightly from the fully closed position, the side of the rotating "plate" that moves forward as it swings open passes additional fuel delivery holes slightly higher in the carburetor throat; these allow more fuel to flow as well as compensating for the reduced vacuum at slight open throttle, thus easing the transition to metering fuel flow through the regular open throttle circuit.

Main open-throttle circuit

As the throttle is progressively opened, the manifold vacuum reduces since there is less restriction on the airflow, reducing the flow through the idle and off-idle circuits. This is where the venturi shape of the carburetor throat comes into play, due to Bernoulli's principle (i.e. as the velocity of a gas increases, its pressure falls). The venturi (sometimes a second or "booster" venturi is placed inside the venturi shaped into the carburetor throat to increase the effect) raises the air velocity, and this high speed and thus low pressure sucks fuel into the airstream through a nozzle located in the center of the venturi.

As the throttle is closed, the airflow through the venturi drops until the lowered pressure is insufficient to maintain this fuel flow, and the idle circuit takes over again, as described above.

Power valve

For open throttle operation a richer mixture will produce more power, prevent detonation, and keep the engine cooler. This is usually addressed with a spring loaded "power valve", which is held shut by engine vacuum. As the throttle opens up, engine vacuum decreases and the spring opens the valve to let more fuel into the main circuit.

Accelerator pump

Similarly, the greater inertia of liquid gasoline, compared to air, means that if the throttle is suddenly opened, the airflow will increase more rapidly than the fuel flow, causing a temporary "lean" condition which causes the engine to "stumble" or "bog down" under acceleration (the opposite of what is normally intended when the throttle is opened). This is remedied by the use of a small mechanical pump (often just a simple plunger which pushes down through a small tube filled with gasoline which feeds into the carburetor throat) which injects an additional amount of fuel as the throttle is opened to cover this lean period; this is usually adjustable for both volume and duration by some means, sometimes just bending the linkage. Often the seals around the moving piston parts of the pump wear out, so that pump output is reduced; this loss of accelerator pump action causes the characteristic stumbling or bogging under acceleration often seen in old, well worn engines until the seals on the pump are replaced. Specialized aftermarket kits are widely available for this purpose. Other variations of pump also exist, such as diaphragm based pumps.

Choke (strangler)

When the engine is cold, fuel vaporizes less readily and tends to condense on the walls of the intake manifold, starving the cylinders of fuel and making the engine difficult to start; thus, a richer mixture (more fuel to air) is required to start and run the engine until it warms up.

To provide the extra fuel, a choke is typically used; this is a device that restricts the flow of air at the entrance to the carburetor, before the venturi. (Although "choke" is the term in common use, it is more properly called a strangler valve, since "choke" is also used to describe the barrel, or venturi, of the carburetor). With this restriction in place, extra vacuum is developed in the carburetor barrel, which pulls fuel through the venturis to supplement the fuel being pulled from the idle and off-idle circuits. This provides the rich mixture required to sustain operation at low engine temperatures.

In addition, the choke is usually connected to a "fast idle cam" or other such device which prevents the throttle from closing fully, which could starve the venturis of vacuum and cause the engine to stall. This also serves as a way to help the engine warm up quickly by idling it at a higher than normal speed. In addition, it increases airflow throughout the intake system which helps to better atomize the cold fuel and smooth out the idle.

In older carbureted cars, the choke was controlled by a cable connected to a knob operated by the driver. In most carbureted cars produced from the mid 1960s onward (in the United States) it is usually automatically controlled by a thermostat, which can be actuated by engine heat, coolant temperature, or a small electrical heating element which causes the choke to open. This device sometimes fails, or the choke may stick in the open or closed position, causing difficulties in starting or failure to run well when warm. Since the choke (when closed) restricts airflow through the carburetor, there is often a linkage to open it somewhat when the throttle is opened past a certain point.

Some carburetors do not have a choke but instead use a mixture enrichment circuit, or an enrichener. Typically used on small engines, notably motorcycles, enricheners work by opening a secondary fuel circuit below the throttle valves. This circuit works exactly like the idle circuit, and when engaged it simply supplies extra fuel when the throttle is closed.

Classic british motorcycles used another type of "cold start device" other than the choke and enrichener. It was called a "tickler". This tickler is simply a spring-loaded rod that, when depressed, manually pushes the float down and allows excess fuel to fill the float bowl and flood the intake tract. It also flooded the outside of the carburetor and the crankcase below and caused a few fires in the process. But, the system worked. The main idea behind these devices is that extra fuel (a rich condition) is necessary to get a "cold" engine started and running for a short period of time. You can do this two ways: restrict the air (choke), or add more fuel (enrichener & tickler). (EFI sytems do this electronically.)

Other elements

The interactions between each circuit may also be affected by various mechanical or air pressure connections and also by temperature sensitive and electrical components. These are introduced for reasons such as response, fuel efficiency or automobile emissions control. Various air bleeds (often chosen from a precisely calibrated range, similarly to the jets) allow air into various portions of the fuel passages to enhance fuel delivery and vaporization. Extra refinements may be included in the carburetor/manifold combination, such as some form of heating to aid fuel vaporization. In order to reduce exhaust nitrogen oxide emissions, many engines contain an exhaust gas recirculation system which is frequently built into, or attached to the carburetor.

Fuel supply

Float chamber

To ensure a ready supply of fuel, the carburetor has a "float chamber" (or "bowl") that contains a quantity of fuel ready for use. It converts fuel from fuel pump pressure to atmospheric pressure. This works similarly to a toilet tank; a float controls an inlet valve. If the float drops, the inlet is opened allowing the fuel to flow under the fuel pump's pressure. The level of fuel maintained in the float bowl can usually be adjusted, whether by a setscrew or by something crude such as bending the arm to which the float is connected. This is usually a critical adjustment (the rule of thumb for the common Chevrolet small block V8 used to be that the float bowl fuel level and the distributor points gap were the only two critical adjustments on the engine) and the proper adjustment is indicated by lines scribed into a window on the float bowl, or a measurement of how far the float hangs below the top of the carburetor when disassembled, or similar. Floats can be made of different materials, such as sheet metal soldered into a hollow shape, or of plastic; hollow floats can spring small leaks and plastic floats can eventually become porous and lose their flotation; in either case the float will fail to float, fuel level will be too high, and the engine will not run well unless the float is replaced. The valve itself becomes worn on it's sides by its motion in its "seat" and will eventually try to close at an angle, and thus fails to shut off the fuel completely; again, this will cause excessive fuel flow and poor engine operation. Conversely, as the fuel evaporates from the float bowl, it leaves sediment, residue, and varnishes behind, which clog the passages and can interfere with the float operation. This is particularly a problem in automobiles operated for only part of the year and left to stand with full float chambers for months at a time; commercial "fuel stabilizer" additives are available that reduce this problem.

Usually, special vent tubes allow air to escape from the chamber as it fills or enter as it empties, maintaining atmospheric pressure within the float chamber; these usually extend into the carburetor throat. Placement of these vent tubes can be somewhat critical to prevent fuel from sloshing out of them into the carburetor, and sometimes they are modified with longer tubing. Note that this leaves the fuel at atmospheric pressure, and therefore it cannot travel into a throat which has been pressurized by a supercharger mounted upstream; in such cases, the entire carburetor must be contained in an airtight pressurized box to operate. This is not necessary in installations where the carburetor is mounted upstream of the supercharger, which is for this reason the more frequent system. However, this results in the supercharger being filled with compressed fuel/air mixture, with a strong tendency to explode should the engine backfire; this type of explosion is frequently seen in drag races, which for safety reasons now incorporate pressure releasing blow-off plates on the intake manifold, breakaway bolts holding the supercharger to the manifold, and shrapnel-catching ballistic nylon blankets surrounding the superchargers.

If the engine must be operated in any orientation (i.e. chain saws), a float chamber cannot work. Instead a diaphragm chamber is used. A flexible diaphragm forms one side of the fuel chamber and is arranged so that as fuel is drawn out into the engine the diaphragm is forced inward by ambient air pressure. The diaphragm is connected to the needle valve and as it moves inward it opens the needle valve to admit more fuel, thus replenishing the fuel as it is consumed. As fuel is replenished the diaphragm moves out due to fuel pressure and a small spring, closing the needle valve. A balanced state is reached which creates a steady fuel reservoir level, which remains constant in any orientation.

Multiple carburetor barrels

Colombo Type 125 "Testa Rossa" engine in a 1961 Ferrari 250TR Spyder with 6 Weber 2 barrel carburetors intaking air through 12 "trumpets" visible on top of the engine, one individually adjustable barrel for each cylinder; the ultimate in tunability.

While low performance carburetors may have only one barrel, most carburetors have more than one venturi, or "barrel", most commonly a two barrel, with 4 barrels being common in higher performance larger displacement engines, to accommodate the higher air flow rate with larger engine displacement. Multi-barrel carburetors can have non-identical primary and secondary barrel{s} of different sizes and calibrated to deliver different air/fuel mixtures; they can be actuated by the linkage or by engine vacuum in "progressive" fashion, so that the secondary barrels do not begin to open until the primaries are almost completely open. This is a desirable characteristic which maximizes airflow through the primary barrel(s) at most engine speeds, thereby maximizing the pressure "signal" from the venturis, but reduces the restriction in airflow at high speeds by adding cross-sectional area for greater airflow. These advantages may not be important in high-performance applications where part throttle operation is irrelevant, and the primaries and secondaries may all open at once, for simplicity and reliability; also, V configuration engines, with two cylinder banks fed by a single carburetor, may be configured with two identical barrels, each supplying one cylinder bank. Similarly, in the widely seen V8 and 4-barrel carburetor combination, there are often two primary and two secondary barrels.

Similarly, multiple carburetors can be mounted on a single engine, often with progressive linkages; three two barrel carburetors ("three deuces") were frequently seen on high performance American V8s before the wide availability of a variety of four barrels, and multiple four barrel carburetors are often now seen on very high performance engines.

Carburetor adjustment

Too much fuel in the fuel-air mixture is referred to as too "rich"; not enough fuel is too "lean". The "mixture" is normally controlled by adjustable screws on an automotive carburetor, or a pilot-operated lever on a propeller aircraft (since mixture is air density (altitude) dependent). The correct air to petrol ratio is 14.6:1, meaning that for each weight unit of petrol, 14.6 units of air will be burned; see also stoichiometry. this is strictly the most efficient but for more power a richer mixture around 11:1 is used and for fuel economy a 18:1 mix. Carburetor adjustment can be checked by measuring the carbon monoxide, hydrocarbon, and oxygen content of the exhaust gases. A more sophisticated way to determine correct mixture, as used in modern fuel injected engines, is by using a lambda sensor to measure the residual oxygen in the exhaust system. The lambda sensor output is fed to the engine management system, that in turn will adjust the amount of injected fuel.

The mixture can also be judged by the state and color of the spark plugs: black, dry sooty plugs indicate a too rich mixture, white to light gray deposits on the plugs indicate a lean mixture. The correct color should be a brownish gray. See also reading spark plugs.

History & development

The carburetor was invented by the Hungarian engineer Donát Bánki in 1893. Frederick William Lanchester of Birmingham, England experimented early on with the wick carburetor in cars. In 1896 Frederick and his brother built the first petrol driven car in England, a single cylinder 5 hp (4 kW) internal combustion engine with chain drive. Unhappy with the performance and power, they re-built the engine the next year into a two cylinder horizontally opposed version using his new wick carburetor design. This version completed a 1,000 mile (1600 km) tour in 1900 incorporating successfully the carburetor as an important step forward in automotive engineering.

Catalytic Carburetors

There are persistent rumours of extremely efficient carburetors. These seem to be true. The basic mechanism is apparently a catalytic carburetor. It would mix fuel fumes with water and air in the presence of heated catalysts such as nickel or platinum. The fuel would break down into methane, alcohols, and other lighter-weight fuels. The lighter-molecular-weight fuels would burn about 1.8 to 3 times as efficiently as vaporized gasoline, with lower emissions and waste. The original catalytic carburetor was introduced to permit farmers to run tractors from kerosene.

The U.S. army used catalytic carburetors in World War II in the North African desert, to achieve substantial logistic surprise against the Germans.

However, it is known that less than two years after commercial introduction of the first catalytic carburetor, in 1932, tetraethyl lead was introduced, to "condition" gasoline's "ethyl content." Also in that time, the price differential between a thermal calorie of gasoline and kerosene was ended. Tetraethyl lead had the effect of poisoning catalytic carburetors. Many modern gasolines appear to have additives for "cleaning" which perform the same effect by producing varnishes or gums in the presence of water, which of course, is not a recommended use.

Gasoline/petrol is an impure mixture of linear heptane and octane and other miscellaneous light alkanes. Commercial gasolines usually contain additives to clean engines, artificially lower evaporation points, and (conjecturally) poison catalytic carburetors (an effect that is certainly real, but might be accidental).

Famed NASCAR mechanic Smokey Yunick spent many years working on a high mileage "vapor carburetor". The detailed operation is not widely disseminated, but the general principle seemed to be heating the fuel with waste engine heat to enhance vaporization and thus efficiency of combustion; it is reported to have been reasonably effective, compared to normal carburetors of the time. However, during the years he was developing it, the average production engine moved from centrally located carburetors to multipoint fuel injection, where the fuel is delivered right to the intake valves and fuel condensation and puddling in the intake manifold are no longer possible, making his design a solution to a problem which no longer existed, and it was never commercially developed.


Some manufacturers of carburetors are/were

Further reading

  • Heldstab, Wayne, "The secret of the super mileage caburator : how they work, how to build them".