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Most carbureted (as opposed to fuel-injected) engines have a single carburetor, though some, primarily higher performance engines, can have multiple carburetors. 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, side-draft carburetors are much more common in performance applications. Small propeller-driven flat airplane engines have the carburetor below the engine ('updraft').
The fundamental function of a carburetor is fairly simple, but the implementation is fairly complex, because a carburetor must provide the ideal fuel/air mixture under a wide variety of different circumstances and engine Rpm. Most carburetors contain equipment to support several different operating modes, called circuits.
A carburetor fundamentally consists of an open pipe, the carburettor's "throat" or "barrel", through which the air passes. The pipe is in the form of a venturi - it narrows in section and then widens again. Just after the narrowest point 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. This in turn affects the engine power and speed. The throttle is connected, via a bowden cable or a set of rods and ball joint s, to the accelerator pedal on a car or the equivalent control on other vehicles or equipment.
When the butterfly valve is closed or nearly closed, the carburetor's idle circuit is in operation. The closed throttle means that a fairly significant vacuum occurs behind the closed butterfly valve. This manifoldIn automotive engineering, an intake manifold or inlet manifold is a part of an engine that supplies the fuel/ air mixture to the cylinders. An exhaust manifold or header collects the exhaust gases from multiple cylinders into one pipe. Due to the sucking vacuumThe article on the vacuum cleaner is located elsewhere. In physics, a vacuum is the absence of matter in a volume of space. A partial vacuum is expressed in units of pressure. The SI unit of pressure is the pascal (abbreviated to Pa in usage). It can also is sufficient to pull fuel and air through small openings placed after the butterfly valve. Only a fairly small amount of air and fuel can pass through in this manner.
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 uncovers extra openings similar to that of the idle circuit. These allow more fuel to flow as well as compensating for the reduced vacuum at slight open throttle.
As the throttle is progressively opened, the manifold vacuum reduces since there is less restriction on the airflow. This reduction in vacuum reduces the flow through the idle and off-idle circuits, so another method of introducing fuel into the airflow is needed.
This is where the venturi shape of the carburetor throat comes into play. The Bernoulli effect shows that as the velocity of a gas increases, its pressure falls. The venturi (sometimes two venturi nested in the same barrel) makes the air reach a higher velocity at the middle than at the ends, and this high speed and thus low pressure in the middle sucks fuel into the airstream through a nozzle (a "jet") located in the center of the throat.
The main circuit requires a reasonable airspeed through the carburetor throat to function, and thus ceases to function during idle, where the idle circuit steps in.