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Fast idle serves as a way to help the engine warm up quickly, and give a more stable idle by increasing airflow throughout the intake system which helps to better atomize the cold fuel. This is generally reported as a 1940s-era product that would allow kerosene to power a gasoline engine requiring lighter hydrocarbons.
In Europe, the sidedraft carburetor replaced downdraft as free space in the engine bay decreased and the use of the -type carburetor and similar units from other manufacturers increased. 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.
- To function correctly under all these conditions, most carburetors contain a complex set of mechanisms to support several different operating modes, called circuits. Forgetting to deactivate the choke once the engine achieved operating temperature would waste fuel and increase emissions.
Bendix-Technico Stromberg 1-barrel downdraft carburetor model BXUV-3, with nomenclature A carburetor or carburettor is a device that mixes air and fuel for 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. They are still common on for , and other equipment. The first carburetor was invented by in 1826. The first person to patent a carburetor for use in a petroleum engine was with his 6 July 1872 patent for a device which mixes fuel with air. A carburetor was among the early patents by 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. In 1885, and developed a float carburetor based on the. The Daimler-Maybach carburetor was copied extensively, leading to patent lawsuits. British courts rejected the Daimler company's claim of priority in favor of 's 1884 spray carburetor used on his. Hungarian and patented a carburetor for a in 1893. In 1896, Frederick and his brother built the first gasoline-driven car in England, a single cylinder 5 hp 3. 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 engines until the late 1980s, when fuel injection became the preferred method. This change was dictated by the requirements of and not due to an inherent inefficiency of carburation. Low-cost commercial vans and 4WDs in Australia continued with carburetors even into the 2000s, the last being the Mitsubishi Express van in 2003. 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 , such as those designed for , though '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. 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 were sold with carburettor engines from its launch in 1991, as were versions of the and launched in 1990 and initially all versions of 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 was the GTI in 1976. The carburetor works on : the faster air moves, the lower its , and the higher its. The accelerator linkage does not directly control the flow of liquid fuel. Instead, it actuates carburetor mechanisms which meter the flow of air being carried into the engine. The speed of this flow, and therefore its static pressure, determines the amount of fuel drawn into the airstream. When carburetors are used in aircraft with piston engines, special are needed to prevent fuel starvation during inverted flight. Later engines used an early form of fuel injection known as a. 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 , each carburetor feeding different chambers of the engine's. Older engines used updraft carburetors, where the air enters from below the carburetor and exits through the top. This had the advantage of never , as any liquid fuel droplets would fall out of the carburetor instead of into the ; it also lent itself to use of an bath , 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 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 carburetor replaced downdraft as free space in the engine bay decreased and the use of the -type carburetor and similar units from other manufacturers increased. Some small propeller-driven aircraft engines still use the updraft carburetor design. The fuel jets are much smaller and fuel flow is limited mainly by the fuel's 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 variable jet carburetors, it was corrected by varying the jet size. For cold starting, a different principle was used in multi-jet carburetors. An airflow resisting valve called a , similar to the throttle valve, was placed upstream of the main jet to reduce the intake manifold pressure and suck additional fuel out of the jets. Fixed-venturi Varying air velocity in the controls the fuel flow; the most common type of carburetor found on cars. Variable-venturi The fuel jet opening is varied by the slide which simultaneously alters air flow. 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 sidedraft and similar models from Hitachi, Zenith-Stromberg and other makers. The UK location of the SU and -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 and other non-UK makes. Other similar designs have been used on some European and a few Japanese automobiles. 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 service. To function correctly under all these conditions, most carburetors contain a complex set of mechanisms to support several different operating modes, called circuits. Basics Cross-sectional schematic of a downdraft carburetor A carburetor basically consists of an open pipe through which the air passes into the 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 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. The throttle is connected, usually through a or a mechanical linkage of rods and joints or rarely by , to the accelerator on a car, a 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 at full throttle. Fuel flow is adjusted by means of precisely calibrated orifices, referred to as jets, in the fuel path. Main open-throttle circuit As the throttle valve is progressively opened, the manifold vacuum is lessened since there is less restriction of the airflow, reducing the fuel flow through the idle and off-idle circuits. This is when the shape of the carburetor throat comes into play, due to i. The venturi increases the air velocity, and this higher speed and thus lower 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 valve is closed, the airflow through the venturi drops until the lowered pressure is insufficient to maintain the fuel flow, and the off-idle circuits take over again, as described above. Bernoulli's principle, which is a function of the velocity of the fluid, is the dominant effect for large openings and large flow rates, but since fluid flow at small scales and low speeds low is dominated by , Bernoulli's principle is ineffective at idle or slow speeds and also 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 producing device for starting cold engines is the choke, which works on the same principle. As the throttle valve opens up, the manifold vacuum decreases and the spring opens the valve to let more fuel into the main circuit. 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. Alternately 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. 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. Accelerator pump Liquid gasoline, being denser than air, is slower than air to applied to it. When the throttle is rapidly opened, airflow through the carburetor increases immediately, faster than the fuel flow rate can increase. This is remedied by the use of a small or 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 the pump output is reduced; this reduction of the accelerator pump shot causes stumbling under acceleration until the seals on the pump are renewed. The accelerator pump can also be used to prime the engine with fuel prior to a cold start. Excessive priming, like an improperly adjusted choke, can cause. 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. Choke 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 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 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 by increasing airflow throughout the intake system which helps to better atomize the cold fuel. In older carbureted cars, the choke was controlled manually by a and pull-knob on the dashboard. For easier, more convenient driving, automatic chokes; first introduced on the , became popular in the late 1950s. These were controlled by a employing a. 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. 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 employing a , heated by the engine coolant. The 'choke' for such as the or 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 for 'enrichment'. Typically used on small engines, notably motorcycles, enrichment works 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. 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. Other elements The interactions between each circuit may also be effected by various mechanical or air pressure connections and also by temperature sensitive and electrical components. These are introduced for reasons such as engine responsiveness, or. 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. This reservoir is constantly replenished with fuel supplied by a. The correct fuel level in the bowl is maintained by means of a float controlling an inlet , in a manner very similar to that employed in a e. 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 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. 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. 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 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. Diaphragm chamber If the engine must be operated in any orientation for example a or a , 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 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. Two barrel and four barrel configurations are commonly used to accommodate the higher air flow rate with large. 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, , and. 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 flow. 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 on an automotive carburetor, or a pilot-operated lever on piston-engined aircraft since the mixture changes with air and therefore altitude. Independent of air density the air to is 14. There are different stoichiometric ratios for other types of fuel. Another method, widely used in aviation, is to measure the , 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. Black, dry, plugs indicate a mixture too rich; white or light gray plugs indicate a lean mixture. On high-performance , 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. 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. This section does not any. Unsourced material may be challenged and. July 2011 A catalytic carburetor mixes fuel vapor with water and air in the presence of heated such as or. This is generally reported as a 1940s-era product that would allow kerosene to power a gasoline engine requiring lighter hydrocarbons. 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, 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 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 above 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. This makes maintenance and tuning difficult. A cutaway view of the intake of the original Fordson tractor including the , , carburetor, and fuel lines. For this purpose, a vaporizer or vaporiser is placed in the intake system. The vaporizer uses heat from the to the fuel. For example, the original and various subsequent Fordson models had vaporizers. 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 in tractors made the use of tractor vaporizing oil obsolete. Mikuni also made racing carburetors for Japanese, British and European cars. Also produced the Zenith-Stromberg carburetors. Retrieved 8 October 2017. Retrieved 8 October 2017. Retrieved 8 October 2017. Retrieved 19 January 2014. Society of Automotive Engineers. Archived from on 17 July 2012. Retrieved 19 January 2014. Society of Automotive Engineers. Archived from on 25 October 2012. Retrieved 19 January 2014. Fundamentals of Motor Vehicle Technology Second ed. Chicago: American technical society.
How to adjust a Briggs and Stratton One Piece Flow Jet Carburetor
British courts rejected the Daimler company's claim of priority in favor of 's 1884 spray carburetor used on his. The Constant Velocity carburetor has a variable throttle closure in the intake air prime before the accelerator pedal operated throttle single jet carburetor pdf. An airflow resisting valve called asimilar to the throttle valve, was placed upstream of the main jet to reduce the intake manifold pressure and suck additional fuel out of the jets. To function correctly under all these conditions, most carburetors contain a complex set of mechanisms to support several different operating modes, called circuits. This section does not any. 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 prime of engine on more expensive vehicles including luxury and sports models. This was designed to provide good mixing and airflow over a range of engine speeds, though the VV carburetor proved problematic in service. Excessive priming, like an improperly adjusted choke, can cause. The most common rich mixture producing device for starting cold engines is the choke, which works on the same principle. The correct fuel level in the bowl is maintained by means of a float controlling an inletin a manner very similar to that employed in a e. The step-up rods are tapered at the bottom end, which extends into the main metering jets.
