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Autolite 4100 Idle Circuit



4100 Idle Circuit

Fuel flows from the float bowl through the main jet up through the idle tube and through a short diagonal passage in the venturi assembly and into the idle passage in the main body. Make sure the small passages in the venturi are clear. Carburetor cleaners may not get it done and you may have to clean them out with thin wire.

Air enter through the idle air bleed and is mixed with the fuel. This air bleed is also a vent to prevent siphoning at off-idle or high speeds and when the engine is stopped. The mixture of air and fuel pass down a diagonal passage in the venturi and through a calibrated restrictor. Additional air is bled into the system through an air bleed located at the bottom of the diagonal passage where teh fuel enters the idle passage in the main body.


Fuel flows down the idle passage in the main body past 2 idle transfer holes. The idle transfer holes act as additional air bleeds at curb idle. The fuel then flows past the pointed tip of the adjusting needle which controls the idle fuel discharge in the primary stage. From the adjusting needle chamber the fuel flows through a short horizontal passage and is discharged below the primary throttle plates.


As you can see it is important to make sure all of these small passages are clear.

You can re-used your old idle mixture screws as long as there is no scoring on the tapered end.

Adjusting your idle mixture

  • Get the engine to operating temperature.
  • Connect a vacuum gauge to the carburetor, or manifold vacuum port.
  • Adjust the idle to manufactures specification.
  • Taking turns with each screw, turn them in 1/4 of a turn, wait a second, then do it again. Do this until the RPM starts to drop.
  • Turn the screws back out 1/4 of a turn.

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