Carburetor Basics

Carburetor Basics
by Don Terrill © -

I hope this article will dispel a few misunderstandings about how a carburetor works.

  1. Needle/Seat - Controlled by the float level. Starts and stops fuel entering bowl.
  2. Float - Rises and falls with fuel level in the bowl, and is meant to control fuel level.
  3. Jets - One of the many factors controlling fuel flow to the engine.
  4. Bowl Vent - Stabilizes air pressure in the bowl.
  5. Air Bleeds - Controls amount air that premixes with fuel.
  6. Emulsifier Tube - Premixes fuel and air.
  7. Venturi - Increases signal to jets.
  8. Booster - Further increases signal to jets, and mixing of fuel and air.

A few things to keep in mind:

  • Fuel enters past the needle/seat, pushed by the fuel pump, but only when the fuel level in the bowl has dropped enough to allow it.
  • The fuel is then pulled through the jets by vacuum created by the boosters. In know I said fuel is pulled through the jets, but if you want to get technical, it's actually pushed by atmospheric pressure via the bowl vent.
  • As the fuel heads toward the booster, it's premixed with air, that comes from, and is metered by the air bleeds. The emulsifier tube does the mixing.
  • The premixed fuel is now pulled out into the middle of the booster, there it is mixed with more air, and then the mixture exits the bottom of the booster to be mixed with more air.
  • Air does not rush in or out of the bowl vent tube, because air in the bowl is not consumed, it's only there to keep consistent pressure on the fuel.

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