Perform a magneto check of each engine at 1,700 RPM as follows: Move ignition switch first to R position and note RPM. Next, move switch back to BOTH, to clear the other set of plugs. Then, move switch to the L position, note RPM and return the switch to the BOTH position. RPM drop should not exceed 150 RPM on either magneto or show greater than 50 RPM differential between magnetos. If there is doubt concerning operation of the ignition system, RPM checks at higher engine speed will usually confirm whether a deficiency exists.
If the magneto check exceeds either of the above limits, both magnetos must be disassembled and inspected in accordance with Requirement 2 of AD/CESSNA 303/8; an entry to this affect is to be made in the Maintenance Release by the pilot.