Watch a video about how the Idle Circuit Works:

4300 Idle Circuit
How the Idle Circuit Works

Using the vacuum from the carburetor bore, fuel is brought up through the idle tube and is mixed with air at the idle air bleed. This air/fuel mixture moves down to the idle mixture screw and out the idle hole into the bore. As rpm is increased a bit more fuel is needed so as the throttle valve opens it uncovers the idle transfer slot, which allows extra air/fuel into the carburetor bore.

Some Autolite 4300 carburetors have an Idle Air Pass System, which provides a more precise way of adjusting the idle rpm. Adjust this screw, along with the idle mixture screws using a vacuum gauge to get the best steady vacuum. Set the idle mixture screws 1st, then use the idle air screw as a final adjustment. You may have to go back and forth between the idle air screw and the idle mixture screws to get it at the smoothest operation.

Along with mixing gas with air, the idle air bleed also prevents siphoning through the idle system at high speeds, or when the engine is shut down. If you are having any excess fuel problems at high speed, or when the engine is shut off, then check that the idle air bleed is clear.

Rough idle, over fueling, higher rpm can all be caused by one of the idle passages being plugged. They are very small.

None of the air bleeds are removable.

4300 Fast IdleThis image may not be exactly like your 4300. Motorcraft changed the 4300 almost every year it was made.

When your engine is cold the rpm needs to be a bit fast than at idle.

Position the fast idle cam so that the idle lever that sits against the cam sits in the middle.

Adjust the screw to the specifications as set forth in your motors manual for the vehicle.

Adjust Idle Mixture
  • Warm up the engine.
  • Set the idle to specification. You cannot adjust the idle mixture if the idle speed is too fast.
  • Adjust by switching between each screw.
  • Turn the screw clockwise 1/4 of a turn at a time until the rpm starts to drop.
  • Turn the screw counter clockwise 1/4 of a turn.
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