A carburetor 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 or multi-choke 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 (US - "under hood") 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.