Recently, I was having trouble setting the Idle Mixture adjustment on an experimental O-540. A friend told me that the range is fairly narrow and that most of the engines he has set were between 1.5 and 2 turns out from full in. With that information, I was able to correctly set it with just a few tries. This is the procedure I used. If there are others with a better method, please chime in here for the benefit of all of us...

Adjusting idle mixture on Lycoming (O-360 & O-540 carbureted) engines:

The idle mixture screw is typically set somewhere between 1.5 and 2 turns out from the full "in" position. Set the screw in the middle of this range. With a warm engine, shut down by pulling the mixture all the way out. As the mixture leans, rpm should increase by 50rpm. Adjust the idle mixture screw in or out within the range until a 50 rpm increase occurs at which point the idle mixture screw is in the correct position.

For reference here is an excerpt from Lycomings Operations Manual regarding Idle Speed and Idle Mixture Adjustment:

e. Idle Speed and Mixture Adjustment.
(1) Start the engine and warm up in the usual manner until oil and cylinder head temperatures are
normal.
(2)Check magnetos. If the mag-drop is normal, proceed with idle adjustment.
(3) Set throttle stop screw so that the engine idles at the airframe manufacturers recommended
idling RPM. If the RPM changes appreciably after making idle mixture adjustment during the
succeeding steps, readjust the idle speed to the desired RPM.
(4) When the idling speed has been stabilized, move the cockpit mixture control lever with a smooth,
steady pull toward the Idle Cut-Off position and observe the tachometer for any change during
the leaning process. Caution must be exercised to return the mixture control to the Full Rich
position before the RPM can drop to a point where the engine cuts out. An increase of more than
50 RPM while leaning out indicates an excessively rich idle mixture. An immediate decrease in
RPM (if not preceded by a momentary increase) indicates the idle mixture is too lean.

If step (4) indicates that the idle adjustment is too rich or too lean, turn the idle mixture
adjustment in direction required for correction, and check this new position by repeating the above
procedure. Make additional adjustments as necessary until a check results in a momentary pick-up
of approximately 50 RPM. Each time the adjustment is changed, the engine should be run up to
2000 RPM to clean the engine before proceeding with the RPM check. Make final adjustment of
the idle speed adjustment to obtain the desired idling RPM with closed throttle. The above method
aims at a setting that will obtain maximum RPM with minimum manifold pressure. In case the
setting does not remain stable, check the idle linkage;any looseness in this linkage would cause
erratic idling. In all cases, allowance should be made for the effect of weather conditions and field
altitude upon idling adjustment.

Blackrock