How does a carburetor work?

Inside a Carburetor

Chainsaw carburetor
Chainsaw carburetor

­­­The carburetor on a chain saw is a good example because it is so straightforward. The carb on a chain saw is simpler than most carbs because it really has only three situations that it has to cover:

  • It has to work when you are trying to start the engine cold.
  • It has to work when the engine is idling.
  • It has to work when the engine is wide open.

­No one operating a chain saw is really interested in any gradations between idle and full throttle, so incremental performance between these two extremes is not very important. In a car the many gradations are important, and this is why a car's carb is a lot more complex.

This short video of the carburetor (5.1 MB) takes you on a quick tour of the carb.

Chainsaw carburetor Chainsaw carburetor
Chainsaw carburetor

Here are the parts of a carb:

  • A carburetor is essentially a tube.
  • There is an adjustable plate across the tube called the throttle plate that controls how much air can flow through the tube. You can see this circular brass plate in photo 1.
  • At some point in the tube there is a narrowing, called the venturi, and in this narrowing a vacuum is created. The venturi is visible in photo 2
  • In this narrowing there is a hole, called a jet, that lets the vacuum draw in fuel. You can see the jet on the left side of the venturi in photo 2. ­

On the next page, learn about carburetor tuning and find out why it's so important.­