Bernoulli's principle is about the behavior of a fluid – a gas or a liquid – as it flows through a pipe or tube. It states that when the pipe narrows or widens, the fluid pressure and speed vary. As the pipe narrows, the speed of the flow goes up and the pressure goes down. When the pipe widens, the fluid flow slows down and the fluid pressure goes up .
This principle is used in a carburetor to mix air with fuel. Air flows through the carburetor in a tube which contains a short section where the diameter narrows then widens again. This section is known as the 'Venturi'. As the air passes through the Venturi, the speed of the air flow increases, but the pressure inside the tube drops. This low pressure air passes over a small inlet which is connected to a gasoline – or petrol – fuel supply, drawing a small amount of fuel vapor into the air flow. The fuel/air mixture then goes to the engine.