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How does a carburetor work?To start, what is the function of a carburetor?It has one main function: mixing an air/fuel mixture.When mixing, the carburetor can mix more or less fuel into the airstream and thus also regulate the power the engine produces at the same time.Carburetors can range from being very straightforward to being extremely complex. Some are very small while some are massive.For instance this is a 5 cc nitro engine. The carburetor is the little black plastic piece on the left:     The carburetor is extremely small and due to its size it is not very complex.Carburetors for trucks or for racing cars can however be very complex and big, for instance the one shown here can run multiple cylinders at the same time.There are a few basic and important parts for the function of a carburetor.In order to be able to mix the fuel consistently in the air, a very small hole is made through which the fuel is sucked due to the lower pressure created by the venturi, which is another important part. A venturi is a constriction created in a tube through which a fluid, either a non-compressible or in this case, a compressible gas, is accelerated and a lower pressure is created. This is called the Venturi Principle.Because the fluid, or in this case gas, is forced through the constriction, its velocity increases. This occurs because the same volume of gas per unit of time has to flow through the restricted area as flows through the non-restricted area.But, as a rule, the molecules have to keep the same amount of energy. Thus, because the velocity of the gas increases, its energy level should rise as well. This can not happen, because no external energy is inserted into the system, so the gas has to lose a different type of energy to make up for this increase. The result of this all is that the pressure will drop when velocity increases. This is shown in the above figure. The level of the for instance water in the figure above, is higher in the non-restricted area compared to the restricted area. This shows the pressure difference as well as that there is a lower pressure where flow is restricted.A carburetor makes use of this principle in order to create a pressure difference substantial enough for the fuel to be pushed by the outside atmosphere pressure into the air stream rushing into the cylinder.In the air stream, more obstructions are placed. In the figure below, a throttle valve as well as a choke valve are shown. The throttle valve works by controlling the volume of air able to pass by the valve. The choke valve also does this. The two are separated from each other because they each have different functions. The throttle valve controls how much air/fuel mixture enters the engine, thus regulating the power and speed. The choke valve is used when starting a cold or old engine, because it can “choke” down the amount of air entering the carburetor, while still allowing fuel to enter the little air flowing by. This mixture is then called a rich mixture, because it has a high fuel/air ratio, leading to easier combustion. This is useful when the engine is cold, because it will start easier.Both valves normally are so-called butterfly valves, because the are a round disc which pivots in the tube. Apparently some people thought this resembled a butterfly.