Carter UT Idle Circuit

Carter UT Idle

It is very important that the small passages you see in the illustration get cleaned out. They are easily clogged.

UT Idle Circuit

Fuel for the idle low speed is drawn from the float bowl A and goes through passage B, past the main jet adjustment C, up the channel and through the idle jet D. The fuel is mixed with air from passage E and then travels through the economizer jet F. Part of the air fuel mixture enters the carburetor bore through port G and the balance travels past the idle mixture screw and into the bore at H.


The idle mixture is controlled by turning the screw clockwise to lean the mixture and counter-clockwise to richen the mixture.

If adjusting the idle mixture screw doesn’t seem to make a difference, then you have something wrong with your carburetor. Could be dirt, or possibly a vacuum leak. Be sure you are adjusting at idle speed, or it will not work.

Do not try to remove the jets E & F. They are permanent.

Adjusting the Idle Mixture

  • Warm up the engine
  • Adjust the idle speed to specification
  • Turn the screw out a turn
  • Turn the screw in 1/4 turn at a time, waiting a second to let the engine catch up.
  • When the RPM starts to drop, turn the screw back out 1/4 turn.
Updated on 12/08/2020

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