What was found
ENGINE WAS HAVING INTERMITTENT RPM LOSS & POWER LOSS WITH NO EXPLANATION AS TO WHY. ON THE GROUND DURING MAGNETO CHECK IT WOULD SOMETIMES BE NORMAL AND SOMETIMES LOSE RPM AND POWER ENOUGH TO ABORT A FLIGHT. OWNER ORDERED NEW MAGNETOS, HARNESS AND SPARK PLUGS, AND AFTER INSTALLATION OF THOSE ITEMS, GROUND RUN REVEALED NO ABNORMAL ISSUES BY MECHANIC, BUT OWNER WAS COMPLETING GROUND RUN AND SWITCHED TO LEFT MAG WHICH CAUSED THE ENGINE TO NEARLY QUIT. AFTER TAXING BACK TO THE MAINTENANCE HANGAR, THE NR 3 CYLINDER VALVE COVER WAS DENTED FROM THE INSIDE. REMOVED VALVE COVER AND FOUND THE EARS ON THE CYLINDER THAT HOLD THE ROCKER ARM SHAFT IN PLACE TO BE BROKEN OFF AND FLOATING.