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Downdraft -> vertical intake, sidedraft -> horizontal intake

Actually, a piece of tubing makes up the main part of a carburetor, even if it is sometimes cleft and craggy, and through the large amount of components hardly recognizable any more. The tube connects the air-filter with the engine intake funnel. In the case of the downdraft carburetor it is vertically mounted and the sidedraft carburetor, horizontally. The 'diagonaldraft carburetor' is very rare and the 'updraft carburetor' is as old as the hills.

Register carburetor -> two for all, twin carburetor -> two for half

In the case of the single carburetor (see figure 1), a tube is connected with all the cylinders. The register carburetor has a second tube in the same casing, connected with the same amount of cylinders as the first. We also speak of the two-stage-carburetor. If two tubes in the casing are divided between the individual cylinders, they form a twin-carburetor.You can see two of them in figure 3. Thus a twelve-cylinder engine can have six twin carbs. Theoretically it could also have 12 single- or 24 register carbs, in reality however, this does not occur.

Air + fuel spray -> flap- or plunger controlled

Pure, filtered air flows into the funnel, what comes out is a fuel-air mixture. The fuel is present as tiny droplets and not as vapour. The flow is controlled by the (throttle) flap which is linked to the accelerator pedal, found mostly at the end of the funnel. There can also be a movable plunger which is, at least as large as the inside diameter of the funnel, opening it wider or less wide at an angle of exactly 90°. In some motorcycle carburetors it is connected to the accelerator twist-grip by a Bowden cable.

Either by vacuum or charging force through the choke passage

The air flowing through the carburettor can either be sucked in by the engine vacuum, or forced through by the charging pressure. In all cases, a choking passage provides for a difference in pressure. In a fixed installation the air-intake funnel is narrowed down to approx. 80 percent, or through a plunger installed crosswise to the flow-direction, which allows a variable choking passage. As opposed to the choking funnel, this provides a constant pressure difference (constant pressure carburetor).

Float chamber with precise level control

Next to- or surrounding the funnel is a small area where enough fuel is stored to accomodate all possible operating conditions. This is mostly filled by a diaphragm pump driven by the petrol engine (see arrow in figure 1). There were also other supply methods, e.g., electric pumps or the natural force of gravity if the fuel tank is installed above the engine. A constant amount of liquid is of great importance and is kept steady by a float-controlled-valve. Therefore, the small area is also called the 'float chamber'.

Main jet controls fuel flow float chamber-choking funnel

The fuel reaches the funnel through a hole drilled from the float chamber to the funnel. It must be, at least at one point, higher than the level of the float, otherwise the float chamber will lose fuel. If the carburetor has a choke funnel, then the holes opening is at it's narrowest point, nearly always in the middle of the funnel. Through the increasing air-speed, a vacuum is developed, with which the atmospheric pressure moves the petrol towards the funnel. For the exact metering, the hole has a changeable jet (choker), which, in the most important fuel-line, is also called the 'main-jet'.

Partial load lean-, full load, acceleration, cold starting -rich mixture

As a rule, a carburetor does not work together with a Lambda regulation (Lambda = 1), thus, it does not have to ensure a constant mixture composition. However, if you look at the air-ratio diagram, you can deduce, that carburetors, in the partial load area, supply a slightly lean mixture, because of consumption reduction, and at full load a somewhat richer mixture, to achieve the maximum performance. An even higher proportion of fuel is added when accelerating and when a cold engine is idling. Nowadays, cold starting uses the highest amount of fuel with a fuel-air ratio of up to 3:1 as opposed to 14,8:1.

Carburetors with a choke funnel require additional systems

The plunger of the constant pressure carburetor regulates at the same time, apart from the vacuum, the fuel supply by inserting it's tapered needle more, or less deep into the nozzle. It manages best of all, to adjust the right mixing proportions for the individual operating conditions with only one system. On the other hand, carburetors with a constant air-funnel diameter require several additional facilities like e.g., accelerator pumps, idling facilities and, for full load operation, also enrichment facilities. 11/10

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