An automobile is driven by an internal combustion engine which will work properly only if the right amounts of petrol and air are mixed together. The carburetor is the part of the engine where the mixing takes place.
The burning of fuel in the engine is a chemical reaction in which petrol combines with the oxygen of the air to produce water, heat energy and oxides of carbon. A chemically correct mixture should have 15 parts of air to one part of petrol, both by weight. The amount of air then present is just sufficient to burn the petrol completely. If the engine used a mixture with an excess of petrol-a rich mixture-a small amount of unburnt petrol will be present in the exhaust fumes.
A carburetor has to produce the required mixture in varying strengths to suit different engine conditions, such as starting, idling, acceleration, cruising and application of full power. It must be able to pass the correct mixture at all engine speeds and under varying loads, and has to atomize the petrol into a combustible mixture.
Inside the carburetor is a throttle valve which can increase or decrease the amount of mixture passing into the cylinders, which in turn controls power of the engine. This valve is mounted on a spindle which is operated by the accelerator pedal.
A special device called a “strangler” is also incorporated to help in starting the engine in cold weather by allowing an extra-rich mixture. This is commonly referred to as the choke.