A carburetor (American English) or carburettor (British English; see
spelling differences) is a device that mixes air and fuel for internal
combustion engines in the proper ratio for combustion. It is sometimes
colloquially shortened to carb in the UK and North America or carby in
Australia. To carburate or carburet (and thus carburation or
carburetion, respectively) means to mix the air and fuel or to equip
(an engine) with a carburetor for that purpose.
Carburetors have largely been supplanted in the automotive and, to a
lesser extent, aviation industries by fuel injection. They are still
common on small engines for lawn mowers, rototillers and other
2 History and development
4.2 Off-idle circuit
4.3 Main open-throttle circuit
4.4 Power valve
4.5 Accelerator pump
4.7 Other elements
5 Fuel supply
5.1 Float chamber
5.2 Diaphragm chamber
6 Multiple carburetor barrels
8 Feedback carburetors
9 Catalytic carburetors
10 Constant vacuum carburetors
12 List of manufacturers
13 See also
15 Further reading
The word carburetor comes from the French carbure meaning
"carbide". Carburer means to combine with carbon (compare also
carburizing). In fuel chemistry, the term has the more specific
meaning of increasing the carbon (and therefore energy) content of a
fluid by mixing it with a volatile hydrocarbon.
History and development
The first carburetor was invented by
Samuel Morey in 1826. Later,
Enrico Bernardi developed another carburetor at the University of
Padua in 1882, for his Motrice Pia, the first petrol combustion engine
(one cylinder, 121.6 cc) prototyped on 5 August 1882.
A carburetor was among the early patents by
Karl Benz (1888) as he
developed internal combustion engines and their components.
Early carburetors were of the surface type, in which air is combined
with fuel by passing over the surface of gasoline.
Wilhelm Maybach and
Gottlieb Daimler developed a float
carburetor based on the atomizer nozzle. The Daimler-Maybach
carburetor was copied extensively, leading to patent lawsuits. British
courts rejected the Daimler company's claim of priority in favor of
Edward Butler's 1884 spray carburetor used on his Petrol Cycle.
János Csonka and
Donát Bánki patented a
carburetor for a stationary engine in 1893.
Frederick William Lanchester of Birmingham, England, experimented with
the wick carburetor in cars. In 1896, Frederick and his brother built
the first gasoline-driven car in England, a single cylinder 5 hp
(3.7 kW) internal combustion engine with chain drive. Unhappy
with the car's performance and power, they re-designed the engine the
following year using two horizontally-opposed cylinders and a newly
designed wick carburetor.
Carburetors were the common method of fuel delivery for most US-made
gasoline engines until the late 1980s, when fuel injection became the
preferred method. This change was mostly dictated by the
requirements of catalytic converters and not due to an inherent
inefficiency of carburation. A catalytic converter requires more
precise control over the fuel / air mixture in order to accurately
control the amount of oxygen in the exhaust gases. In the U.S. market,
the last cars using carburetors were:
1990 (General public) : Oldsmobile Custom Cruiser, Buick Estate
Wagon, Cadillac Brougham,
Honda Prelude (Base Model), Subaru Justy
1991 (Police) :
Ford Crown Victoria
Ford Crown Victoria Police Interceptor with the
5.8 L (351 cu in) V8 engine.
1991 (SUV) : Jeep Grand Wagoneer with the AMC 360 cu in
(5.9 L) V8 engine.
1993 (Light Truck) : Mazda B2200
1994 (Light truck) : Isuzu
In Australia, some cars continued to use carburetors well into the
1990s; these included the
Honda Civic (1993), the Ford Laser (1994),
the Mazda 323 and Mitsubishi Magna sedans (1996), the Daihatsu Charade
(1997), and the Suzuki Swift (1999). Low-cost commercial vans and 4WDs
in Australia continued with carburetors even into the 2000s, the last
being the Mitsubishi Express van in 2003. Elsewhere,
Lada cars used carburetors until 2006. Many motorcycles still
use carburetors for simplicity's sake, since a carburetor does not
require an electrical system to function. Carburetors are also still
found in small engines and in older or specialized automobiles, such
as those designed for stock car racing, though NASCAR's 2011 Sprint
Cup season was the last one with carbureted engines; electronic fuel
injection was used beginning with the 2012 race season in Cup.
In Europe, carburetor-engined cars were being gradually phased out by
the end of the 1980s in favor of fuel injection, which was already the
established type of engine on more expensive vehicles including luxury
and sports models. EEC legislation required all vehicles sold and
produced in member countries to have a catalytic converter after
December 1992. This legislation had been in the pipeline for some
time, with many cars becoming available with catalytic converters or
fuel injection from around 1990. However, some versions of the Peugeot
106 were sold with carburettor engines from its launch in 1991, as
were versions of the
Renault Clio and
Nissan Primera (launched in
1990) and initially all versions of
Ford Fiesta range except the XR2i
when it was launched in 1989. Luxury car manufacturer Mercedes-Benz
had been producing mechanically fuel-injected cars since the early
1950s, while the first mainstream family car to feature fuel injection
Volkswagen Golf GTI in 1976. Ford's first fuel-injected car
Ford Capri RS 2600 in 1970.
General Motors launched its first
fuel-injected car in 1957 as an option available for the first
generation Corvette. Saab switched to fuel injection across its whole
range from 1982.
The carburetor works on Bernoulli's principle: the faster air moves,
the lower its static pressure, and the higher its dynamic pressure.
The throttle (accelerator) linkage does not directly control the flow
of liquid fuel. Instead, it actuates carburetor mechanisms which meter
the flow of air being pushed into the engine. The speed of this flow,
and therefore its pressure, determines the amount of fuel drawn into
When carburetors are used in aircraft with piston engines, special
designs and features are needed to prevent fuel starvation during
inverted flight. Later engines used an early form of fuel injection
known as a pressure carburetor.
Most production carbureted engines, as opposed to fuel-injected, have
a single carburetor and a matching intake manifold that divides and
transports the air fuel mixture to the intake valves, though some
engines (like motorcycle engines) use multiple carburetors on split
heads. Multiple carburetor engines were also common enhancements for
modifying engines in the USA from the 1950s to mid-1960s, as well as
during the following decade of high-performance muscle cars, fueling
different chambers of the engine's intake manifold.
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.
Beginning in the late 1930s, downdraft carburetors were the most
popular type for automotive use in the United States. In Europe, the
sidedraft carburetors replaced downdraft as free space in the engine
bay decreased and the use of the SU-type carburetor (and similar units
from other manufacturers) increased. Some small propeller-driven
aircraft engines still use the updraft carburetor design.
Outboard motor carburetors are typically sidedraft, because they must
be stacked one on top of the other in order to feed the cylinders in a
vertically oriented cylinder block.
1979 Evinrude Type I marine sidedraft carburetor
The main disadvantage of basing a carburetor's operation on
Bernoulli's Principle is that, being a fluid dynamic device, the
pressure reduction in a Venturi tends to be proportional to the square
of the intake air speed. The fuel jets are much smaller and limited
mainly by viscosity, so that the fuel flow tends to be proportional to
the pressure difference. So jets sized for full power tend to starve
the engine at lower speed and part throttle. Most commonly this has
been corrected by using multiple jets. In SU and other movable jet
carburetors, it was corrected by varying the jet size. For cold
starting, a different principle was used in multi-jet carburetors. A
flow resisting valve called a choke, similar to the throttle valve,
was placed upstream of the main jet to reduce the intake pressure and
suck additional fuel out of the jets.
in which the varying air velocity in the Venturi alters the fuel flow;
this architecture is employed in most carburetors found on cars.
in which the fuel jet opening is varied by the slide (which
simultaneously alters air flow). In "constant depression" carburetors,
this is done by a vacuum operated piston connected to a tapered needle
which slides inside the fuel jet. A simpler version exists, most
commonly found on small motorcycles and dirt bikes, where the slide
and needle is directly controlled by the throttle position. The most
common variable Venturi (constant depression) type carburetor is the
SU carburetor and similar models from Hitachi,
Zenith-Stromberg and other makers. The UK location of the SU and
Zenith-Stromberg companies helped these carburetors rise to a position
of domination in the UK car market, though such carburetors were also
very widely used on Volvos and other non-UK makes. Other similar
designs have been used on some European and a few Japanese
automobiles. These carburetors are also referred to as "constant
velocity" or "constant vacuum" carburetors. An interesting variation
was Ford's VV (Variable Venturi) carburetor, which was essentially a
fixed Venturi carburetor with one side of the Venturi hinged and
movable to give a narrow throat at low rpm and a wider throat at high
rpm. This was designed to provide good mixing and airflow over a range
of engine speeds, though the VV carburetor proved problematic in
A high performance 4-barrel carburetor
Under all engine operating conditions, 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 if air and gasoline (petrol) were ideal
fluids; in practice, however, their deviations from ideal behavior due
to viscosity, fluid drag, inertia, etc. require a great deal of
complexity to compensate for exceptionally high or low engine speeds.
A carburetor must provide the proper fuel/air mixture across a wide
range of ambient temperatures, atmospheric pressures, engine speeds
and loads, and centrifugal forces:
Idling or slow-running
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.
Cross-sectional schematic of a downdraft carburetor
A carburetor basically consists of an open pipe 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 called the throttle valve — 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, a throttle level in an aircraft 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 and at other places where pressure will
be lowered when not running on full throttle. Fuel flow is adjusted by
means of precisely calibrated orifices, referred to as jets, in the
As the throttle is opened up slightly from the fully closed position,
the throttle plate uncovers additional fuel delivery holes behind the
throttle plate where there is a low pressure area created by the
Valve blocking air flow; these allow more fuel to flow
as well as compensating for the reduced vacuum that occurs when the
throttle is opened, thus smoothing 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 is
lessened 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 increases, pressure falls). The
Venturi raises the air velocity, and this high speed and thus low
pressure sucks fuel into the airstream through a nozzle or nozzles
located in the center of the Venturi. Sometimes one or more additional
booster Venturis are placed coaxially within the primary Venturi to
increase the effect.
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.
Bernoulli's principle, which is a function of the velocity of the
fluid, is a dominant effect for large openings and large flow rates,
but since fluid flow at small scales and low speeds (low Reynolds
number) is dominated by viscosity,
Bernoulli's principle is
ineffective at idle or slow running and in the very small carburetors
of the smallest model engines. Small model engines have flow
restrictions ahead of the jets to reduce the pressure enough to suck
the fuel into the air flow. Similarly the idle and slow running jets
of large carburetors are placed after the throttle valve where the
pressure is reduced partly by viscous drag, rather than by Bernoulli's
principle. The most common rich mixture device for starting cold
engines was the choke, which works on the same principle.
For open throttle operation a richer mixture will produce more power,
prevent pre-ignition 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, the vacuum decreases
and the spring opens the valve to let more fuel into the main circuit.
On two-stroke engines, the operation of the power valve is the reverse
of normal — it is normally "on" and at a set rpm it is turned "off".
It is activated at high rpm to extend the engine's rev range,
capitalizing on a two-stroke's tendency to rev higher momentarily when
the mixture is lean.
Alternative to employing a power valve, the carburetor may utilize a
metering rod or step-up rod system to enrich the fuel mixture under
high-demand conditions. Such systems were originated by Carter
Carburetor in the 1950s for the primary two Venturis
of their four barrel carburetors, and step-up rods were widely used on
most 1-, 2-, and 4-barrel Carter carburetors through the end of
production in the 1980s. The step-up rods are tapered at the bottom
end, which extends into the main metering jets. The tops of the rods
are connected to a vacuum piston or a mechanical linkage which lifts
the rods out of the main jets when the throttle is opened (mechanical
linkage) or when manifold vacuum drops (vacuum piston). When the
step-up rod is lowered into the main jet, it restricts the fuel flow.
When the step-up rod is raised out of the jet, more fuel can flow
through it. In this manner, the amount of fuel delivered is tailored
to the transient demands of the engine. Some 4-barrel carburetors use
metering rods only on the primary two Venturis, but some use them on
both primary and secondary circuits, as in the Rochester Quadrajet.
Liquid gasoline, being denser than air, is slower than air to react to
a force applied to it. When the throttle is rapidly opened, airflow
through the carburetor increases immediately, faster than the fuel
flow rate can increase. This transient oversupply of air causes a lean
mixture, which makes the engine misfire (or "stumble")—an effect
opposite to that which was demanded by opening the throttle. This is
remedied by the use of a small piston or diaphragm pump which, when
actuated by the throttle linkage, forces a small amount of gasoline
through a jet into the carburetor throat. This extra shot of fuel
counteracts the transient lean condition on throttle tip-in. Most
accelerator pumps are adjustable for volume or duration by some means.
Eventually, the seals around the moving parts of the pump wear such
that pump output is reduced; this reduction of the accelerator pump
shot causes stumbling under acceleration until the seals on the pump
The accelerator pump is also used to prime the engine with fuel prior
to a cold start. Excessive priming, like an improperly adjusted choke,
can cause flooding. This is when too much fuel and not enough air are
present to support combustion. For this reason, most carburetors are
equipped with an unloader mechanism: The accelerator is held at wide
open throttle while the engine is cranked, the unloader holds the
choke open and admits extra air, and eventually the excess fuel is
cleared out and the engine starts.
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. A richer mixture is also easier to ignite.
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. With this restriction in place, extra vacuum is
developed in the carburetor barrel, which pulls extra fuel through the
main metering system 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 can be connected to a cam (the fast idle cam)
or other such device which prevents the throttle plate from closing
fully while the choke is in operation. This causes the engine to idle
at a higher speed. Fast idle serves as a way to help the engine warm
up quickly, and give a more stable idle while cold by increasing
airflow throughout the intake system which helps to better atomize the
In older carbureted cars, the choke was controlled manually by a
Bowden cable and pull-knob on the dashboard. For easier, more
convenient driving, automatic chokes; first introduced on the 1932
Oldsmobile, became popular in the late 1950s. These were controlled by
a thermostat employing a bimetallic spring. When cold, the spring
would contract, closing the choke plate. Upon startup the spring would
be heated by engine coolant, exhaust heat or an electric heating coil.
As it was heated, the spring would slowly expand and open the choke
plate. A choke unloader is a linkage arrangement that forces the choke
open against its spring when the vehicle's accelerator is moved to the
end of its travel. This provision allows a "flooded" engine to be
cleared out so that it will start.
Forgetting to deactivate the choke once the engine achieved operating
temperature would waste fuel and increase emissions. To meet
increasingly stringent emission requirements, some cars that still
retained manual chokes (from around 1980, depending on market) began
to have choke opening automatically controlled by a thermostat
employing a bimetallic spring, heated by the engine coolant.
The 'choke' for constant-depression carburettors such as the SU or
Stromberg does not use a choke valve in the air circuit but instead
has a mixture enrichment circuit to increase fuel flow by opening the
metering jet further or by opening an additional fuel jet or
'enrichment'. Typically used on small engines, notably motorcycles,
enrichments 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
Classic British motorcycles, with side-draft slide-throttle
carburetors, used another type of "cold start device", called a
"tickler". This 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. If the "tickler" is held down
too long it also floods the outside of the carburetor and the
crankcase below, and is therefore a fire hazard.
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 such as an early fuel
Holley "Visi-Flo" model #1904 carburetors from the 1950s, factory
equipped with transparent glass bowls.
To ensure a ready mixture, the carburetor has a "float chamber" (or
"bowl") that contains a quantity of fuel at near-atmospheric pressure,
ready for use. This reservoir is constantly replenished with fuel
supplied by a fuel pump. The correct fuel level in the bowl is
maintained by means of a float controlling an inlet valve, in a manner
very similar to that employed in a cistern (e.g. a toilet tank). As
fuel is used up, the float drops, opening the inlet valve and
admitting fuel. As the fuel level rises, the float rises and closes
the inlet valve. 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, and the proper adjustment is indicated by lines
inscribed 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
brass 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 unless the
float is replaced. The valve itself becomes worn on its 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.
The fuel stored in the chamber (bowl) can be a problem in hot
climates. If the engine is shut off while hot, the temperature of the
fuel will increase, sometimes boiling ("percolation"). This can result
in flooding and difficult or impossible restarts while the engine is
still warm, a phenomenon known as "heat soak". Heat deflectors and
insulating gaskets attempt to minimize this effect. The Carter
Thermo-Quad carburetor has float chambers manufactured of insulating
plastic (phenolic), said to keep the fuel 20 degrees Fahrenheit (11
degrees Celsius) cooler.
Usually, special vent tubes allow atmospheric pressure to be
maintained in the float chamber as the fuel level changes; these tubes
usually extend into the carburetor throat. Placement of these vent
tubes is 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 blankets made from nylon or kevlar
surrounding the superchargers.
If the engine must be operated in any orientation (for example a chain
saw or a model airplane), a float chamber is not suitable. 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
Holley model #2280 2-barrel carburetor
Colombo Type 125 "Testa Rossa" engine in a 1961 Ferrari 250TR Spider
with six Weber two-barrel carburetors inducting air through 12 air
horns; one individually adjustable barrel for each cylinder.
While basic carburetors have only one Venturi, many carburetors have
more than one Venturi, or "barrel". Two barrel and four barrel
configurations are commonly used to accommodate the higher air flow
rate with large 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. In the widely seen V8 and 4-barrel carburetor combination, there
are often two primary and two secondary barrels.
The first four-barrel carburetors, with two primary bores and two
secondary bores, were the Carter WCFB and identical Rochester 4GC
simultaneously introduced on the 1952 Cadillacs, Oldsmobile 98,
Oldsmobile Super 88 and Buick Roadmaster. Oldsmobile referred the new
carburetor as the “Quadri-Jet” (original spelling) while Buick
called it the “Airpower”.
The spread-bore four-barrel carburetor, first released by Rochester in
the 1965 model year as the "Quadrajet" has a much
greater spread between the sizes of the primary and secondary throttle
bores. The primaries in such a carburetor are quite small relative to
conventional four-barrel practice, while the secondaries are quite
large. The small primaries aid low-speed fuel economy and
driveability, while the large secondaries permit maximum performance
when it is called for. To tailor airflow through the secondary
Venturis, each of the secondary throats has an air valve at the top.
This is configured much like a choke plate, and is lightly
spring-loaded into the closed position. The air valve opens
progressively in response to engine speed and throttle opening,
gradually allowing more air to flow through the secondary side of the
carburetor. Typically, the air valve is linked to metering rods which
are raised as the air valve opens, thereby adjusting secondary fuel
Multiple carburetors can be mounted on a single engine, often with
progressive linkages; two four-barrel carburetors (often referred to
as "dual-quads") were frequently seen on high performance American
V8s, and multiple two barrel carburetors are often now seen on very
high performance engines. Large numbers of small carburetors have also
been used (see photo), though this configuration can limit the maximum
air flow through the engine due to the lack of a common plenum; with
individual intake tracts, not all cylinders are drawing air at once as
the engine's crankshaft rotates.
The fuel and air mixture is too rich when it has an excess of fuel,
and too lean when there is not enough. The mixture is adjusted by one
or more needle valves on an automotive carburetor, or a pilot-operated
lever on piston-engined aircraft (since the mixture changes with air
density and therefore altitude). Independent of air density the
(stoichiometric) air to gasoline ratio is 14.7:1, meaning that for
each mass unit of gasoline, 14.7 mass units of air are required. There
are different stoichiometric ratios for other types of fuel.
Ways to check carburetor mixture adjustment include: measuring the
carbon monoxide, hydrocarbon, and oxygen content of the exhaust using
a gas analyzer, or directly viewing the color of the flame in the
combustion chamber through a special glass-bodied spark plug sold
under the name "Colortune"; the flame color of stoichiometric burning
is described as a "Bunsen blue", turning to yellow if the mixture is
rich and whitish-blue if too lean. Another method, widely used in
aviation, is to measure the exhaust gas temperature, which is close to
maximum for an optimally adjusted mixture and drops off steeply when
the mixture is either too rich or too lean.
The mixture can also be judged by removing and scrutinizing the spark
plugs. Black, dry, sooty plugs indicate a mixture too rich; white or
light gray plugs indicate a lean mixture. A proper mixture is
indicated by brownish-gray/straw-coloured plugs.
On high-performance two-stroke engines, the fuel mixture can also be
judged by observing piston wash.
Piston wash is the color and amount
of carbon buildup on the top (dome) of the piston. Lean engines will
have a piston dome covered in black carbon, and rich engines will have
a clean piston dome that appears new and free of carbon buildup. This
is often the opposite of intuition. Commonly, an ideal mixture will be
somewhere in-between the two, with clean dome areas near the transfer
ports but some carbon in the center of the dome.
When tuning two-strokes It is important to operate the engine at the
rpm and throttle input that it will most often be operated at. This
will typically be wide-open or close to wide-open throttle. Lower RPM
and idle can operate rich/lean and sway readings, due to the design of
carburetors to operate well at high air-speed through the Venturi and
sacrifice low air-speed performance.
Where multiple carburetors are used the mechanical linkage of their
throttles must be properly synchronized for smooth engine running and
consistent fuel/air mixtures to each cylinder.
In the 1980s, many American-market vehicles used "feedback"
carburetors that dynamically adjusted the fuel/air mixture in response
to signals from an exhaust gas oxygen sensor to provide a
stoichiometric ratio to enable the optimal function of the catalytic
converter. Feedback carburetors were mainly used because they were
less expensive than fuel injection systems; they worked well enough to
meet 1980s emissions requirements and were based on existing
carburetor designs. Frequently, feedback carburetors were used in
lower trim versions of a car (whereas higher specification versions
were equipped with fuel injection). However, their
complexity compared to both non-feedback carburetors and to fuel
injection made them problematic and difficult to service.[citation
needed] Eventually falling hardware prices and tighter emissions
standards caused fuel injection to supplant carburetors in new-vehicle
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A catalytic carburetor mixes fuel vapor with water and air in the
presence of heated catalysts such as nickel or platinum. This is
generally reported as a 1940s-era product that would allow kerosene to
power a gasoline engine (requiring lighter hydrocarbons). However
reports are inconsistent; commonly they are included in descriptions
of "200 MPG carburetors" intended for gasoline use. There seems to be
some confusion with some older types of fuel vapor carburetors (see
vaporizors below). There is also very rarely any useful reference to
real-world devices. Poorly referenced material on the topic should be
viewed with suspicion.
Constant vacuum carburetors
Constant vacuum carburetors, also called variable choke carburetors
and constant velocity carburetors, are carburetors where the throttle
cable was connected directly to the throttle cable plate. Pulling the
cord caused raw gasoline to enter the carburetor, creating a large
emission of hyrdocarbons.
The Constant Velocity carburetor has a variable throttle closure in
the intake air stream before the accelerator pedal operated throttle
plate. This variable closure is controlled by intake manifold
pressure/vacuum. This pressure controlled throttle provides relatively
even intake pressure throughout the engine's speed and load ranges.
The most common design of the CV carburetor would be that of the SU or
Solex, among others, which use a cylindrical closure that is operated
by a diaphram. The cylinder and diaphram are connected together with
the fuel metering rod to provide fuel in direct relation to air flow.
To provide more smooth operation and more even intake pressure, the
diaphram is viscous dampened. These carburetors allowed for very good
drivability and fuel efficiency. They are also widely adjustable for
best performance and efficiency. (See Variable Venturi Carburetors
Drawbacks of the CV carburetor include that it is limited to a single
barrel, side draft design. This limited its use to mostly inline
engines and also made it impractical for large displacement engines.
The throttle linkage required to install 2 or more CV carbs on an
engine is complex and proper adjustment is critical for even air/fuel
distribution. This makes maintenance and tuning difficult.
A cutaway view of the intake of the original Fordson tractor
(including the intake manifold, vaporizer, carburetor, and fuel
Internal combustion engines can be configured to run on many kinds of
fuel, including gasoline, kerosene, tractor vaporizing oil (TVO),
vegetable oil, diesel fuel, biodiesel, ethanol fuel (alcohol), and
Multifuel engines, such as petrol-paraffin engines, can
benefit from an initial vaporization of the fuel when they are running
less volatile fuels. For this purpose, a vaporizer (or vaporiser) is
placed in the intake system. The vaporizer uses heat from the exhaust
manifold to vaporize the fuel. For example, the original Fordson
tractor and various subsequent Fordson models had vaporizers. When
Henry Ford & Son Inc designed the original Fordson (1916), the
vaporizer was used to provide for kerosene operation. When TVO became
common in various countries (including the United Kingdom and
Australia) in the 1940s and 1950s, the standard vaporizers on Fordson
models were equally useful for TVO. Widespread adoption of diesel
engines in tractors made the use of tractor vaporizing oil obsolete.
List of manufacturers
AMAL, producer of carburetors and hand controls for British
motorcycles and light industrial engines
Autolite, a division of the
Ford Motor Company
Ford Motor Company from 1967 to 1973
Bendix Stromberg and Bendix Technico carburetors used on aircraft and
vehicles made by Chrysler, IHC, Ford, GM, AMC, and Studebaker
Carter, used on numerous makes of vehicles, including those made by
Chrysler, IHC, Ford, GM, AMC, and Studebaker, as well as on industrial
and agricultural equipment and small engines.
Dell'Orto carburetors from Italy, used on cars and motorcycles
Edelbrock performance carburetors
Hitachi, found on Japanese vehicles
Holley, with usage as broad as Carter and Weber
Keihin, a keiretsu group company affiliated with Honda
Mikuni, common on Japanese motorcycles, especially in the 1980s.
Mikuni also made racing carburetors for Japanese, British and European
cars. Original equipment on Mitsubishi engines
Reece Fish, in Volkswagen, Austin Mini, Morris Mini
Rochester Products Division, a
General Motors subsidiary; also sold
Magneti Marelli carburetors under license)
Solex – French carburetors, owned by Weber
SU Carburettors, widely used on British Commonwealth and
Villiers, used on UK motorcycles and small engines
Walbro and Tillotson carburetors for small engines
Weber carburetor, Italian, now made in Spain, owned by Magneti Marelli
Zenith, used on Austin cars. Also produced the Zenith-Stromberg
^ Beale, Paul; Partridge, Eric (2003), Shorter Slang Dictionary,
Routledge, p. 60, ISBN 9781134879519
^ "American Heritage Dictionary". Answers.com. Retrieved 8 October
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^ "Carbueetoe". Google.com. Retrieved 8 October 2017.
^ Inventors and Inventions. Marshall Cavendish. 2008. p. 91.
ISBN 9780761477617. Retrieved 19 January 2014.
^ Webster's Revised Unabridged Dictionary, 1913
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Automotive Engineers. p. 276. ISBN 978-0-7680-0800-5.
^ Carlisle, Rodney (2005), Scientific American Inventions and
Discoveries: All the Milestones in Ingenuity—From the Discovery of
Fire to the Invention of the Microwave Oven, John Wiley & Sons,
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Science Guide for the Traveler. Springer.
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July 2012. Retrieved 19 January 2014.
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NASCAR takes 'really big step' with
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Packer, Ed (July 1953). "Know Your
Carburetor – what it is, what it
does". Popular Mechanics. 100 (1): 181–184.
American Technical Society. (1921).
Automobile engineering; A general
reference work. Chicago: American technical society.
Lind, W. L. (1920). Internal-combustion engines; Their principles and
applications to automobile, aircraft, and marine purposes. Boston:
Hutton, F. R. (1908). The gas-engine. A treatise on the
internal-combustion engine using gas, gasoline, kerosene, alcohol, or
other hydrocarbon as source of energy. New York: Wiley.
U.S. Patent 610,040 —
Carburetor — Henry Ford
*[//www.google.com/patents/US1204901 U.S. Patent 1,204,901 Carburetor
Antoine Prosper Plaut]
U.S. Patent 1,750,354 —
Carburetor — Charles Nelson Pogue
U.S. Patent 1,938,497 —
Carburetor — Charles Nelson Pogue
U.S. Patent 1,997,497 —
Carburetor — Charles Nelson Pogue
U.S. Patent 2,026,798 —
Carburetor — Charles Nelson Pogue
U.S. Patent 2,214,273 —
Carburetor — J. R. Fish
U.S. Patent 2,982,528 — Vapor fuel system — Robert S. Shelton
U.S. Patent 4,177,779 — Fuel economy system for an internal
combustion engine — Thomas H. W.
G.B. Рatent 11119 — Mixing chamber — Donát Bánki
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