Can somebody explain to me what those little holes do in the carburetor? The carb in question is Keihin PE 28.

I just want to know what they do.

Thank you.


In a carburetor, there are vertical pipes leading from the venturi (where the air flows through) down into the gas reservoir. The nozzles typically are brass screws with a central drilling at the lower end of the pipes, which restrict the gas flow.

The holes in the image lead horizontally from the inlet to the upper third of the vertical pipes. So some air flows through this holes into the vertical pipes, where it carries away the gas, into the motor.

Your carburetor has three holes:

  • The one at the 6 o'clock position leads straight to the main pipe with the main nozzle. This is located centrally below the venturi.

  • The big brass screw on the right sets the idle position of the throttle, and the small brass screw below sets the idle mixture. In fact, this small screw restricts the horizontal air flow through the hole at the 5 o'clock position to a pipe with a small idle nozzle.

  • Finally, the vertical, shiny metallic part with the black rubber in the background is the choke, which adds some extra fuel when the motor is cold. Guess, the choke has its own pipe with nozzle, and the hole at the 9 o'clock position leads there.


Sometimes, the horizontal air channels have to go around a corner. Since one can't drill around a corner, one drills two intersecting holes, and closes the opening of the unwanted hole, for example with this brass ball right of the 6 o'clock hole.

It would guess that this closed hole belongs to the idle mixture system. While the screw for the mixture is relatively near to the front of the carburetor, the output of this system should be more at the rear.

You can open the reservoir, there you can typically see where all the channels go: enter image description here

  • Ok, then how about the brass ball thing between the holes there? May 6 '17 at 10:33

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.