idle mixture screw adjustment

Idle mixture screw adjustment – optimize your Mustang’s carb

In order to optimize the fuel/air mixture on your Mustang’s carburetor, you need to correctly adjust the idle mixture screws to make the mix leaner or richer. Learn all you need to know about idle mixture screw adjustment.

The symptoms of running too rich or too lean are easy to see.

Carb running too rich:

  • Blackened, sooty spark plug tips
  • Blackened tail pipe
  • In some cases, black smoke while accelerating
  • Soggy performance
  • Possibly flooding

Carb running too lean:

  • Whitish colored spark plug tips
  • Less sense of power upon first acceleration
  • White or light gray muffler end pipes

Important note: the idle mixture screws only affect idle richness. They have nothing to do with the general performance when accelerating. As soon as you hit the gas pedal, they have no effect.

How to adjust the idle on a 65 Mustang

The idle mixture screws can be found at the front of the carburetor.

On most carbs, and certainly on the 2100 Autolite or Motorcraft carburetors most stock Mustangs were originally equipped with, proceed with idle mixture screw adjustment as follows:

  • To make the mixture LEANER, turn both screws IN (meaning inwards or clockwise). This will admit more air into the carb.
  • To make the mixture RICHER, turn both screws OUT (meaning outwards or counterclockwise).

If you want to get serious about making this adjustment, it’s best to use a vacuum gauge. Ideally, you want to reach the adjustment that gives you the highest vacuum reading (also known as ‘best lean idle’).


  • Both idle screws should be the exact same number of turns out at all times.
  • NEVER screw the idle mixture screws in all the way and too tightly. If you want to start from scratch, make sure the screws are lightly seated when you turn them in.
  • Some carburetors have a reverse idle adjustment (some 2300, 4150 and 4160 Holley carbs, for example). On these carbs, the air/fuel mixture is made leaner by turning counterclockwise (or OUT) and made richer by turning clockwise (or IN). When in doubt, look up and consult the factory manual for your specific carburetor.

On the stock Autolite 2100 carb, you’ll find the idle screws at the front bottom of your carb:

Idle screw autolite 2100

On a Holley Street Avenger 500cfm, look for the idle screws at both sides of the carb:

holley idle screw

Did this post help you? Do you have experience with this subject? Share your thoughts or ask a question in the comments section!

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