I had a similar situation arise on the Aztec (on one side only) and the cause was found to be two oiled plugs in the same cylinder, meaning that the aircraft was producing no more than (and probably considerably less than) five/sixths power. Of course in a four banger, this would be three-quarters. If both plugs have oiled (i) it won't show in a mag check and (ii) no amount of running at a lean mixture will clean them.
If static RPM is down you must abandon the flight. If you don't, why bother with power checks, or any other checks, for that matter?
Timothy