What Lycoming have to say on the issue. IO-540 Operators Manual,
A proper magneto check is important. Additional factors, other than the ignition system, affect magneto drop-off. They are load-power output, propeller pitch and mixture strength. The important thing is that the engine runs smoothly because magneto drop-off is affected by the variables listed above. Make the magneto check in accordance with the following procedures.
(I) (Controllable Pitch Propeller.) With propeller in minimum pitch angle, set the engine to produce 50-65% power as indicated by the manifold pressure gage unless otherwise specified in the aircraft manufacturer's manual. Set the mixture control to the full rich position. At these settings, the ignition system and spark plugs must work harder because of the greater pressure within the cylinders. Under these conditions ignition problems, if they exist, will occur. Mag checks at low power settings will only indicate fuel-air distribution quality.
NOTE]Aircraft that are equipped with fixed pitch propellers‘, or not equipped with manifold pressure gage. may check magneto drop-of with engine operating at approximately 2100 - 2200 RPM.
Switch from both magnetos to one and note drop-off’, return to both until engine regains speed and switch to the other magneto and note drop-off, then return to both. Drop-off should not exceed 175 RPM and should not exceed 50 RPM between magnetos. A smooth drop-off past the normal specification of 175 RPM is usually a sign of a too lean or too rich mixture.
If the RPM drop exceeds 175 RPM, slowly lean the mixture until the RPM peaks. Then retard the throttle to the RPM specified in step (l} for the magneto check and repeat the check. lf the drop-off does not exceed 175 RPM. the difference between the magnetos does not exceed 50 RPM. and the engine is running smoothly, then the ignition system is operating properly. Return the mixture to full rich.
Do not operate on a single magneto For too long a period, a few seconds is usually sufficient to cheek drop-off and will minimize plug fouling
From the Lycoming produced "Flyer" series of articles, note para 4 at bottom of first column.
