What was found
AFTER INITIATING TAKEOFF ROLL BY ADVANCING THROTTLE FULL FORWARD, PILOT NOTED LESS THAN NORMAL POWER AND VERIFIED MAXIMUM RPM ACHIEVED WAS 2000 RPM. PILOT REJECTED TAKEOFF AND RETURNED TO GATE. POST FLIGHT RUN-UP REVEALED ROUGH RUNNING ENGINE ABOVE 1,200 RPM, WHEN IGNITION SWITCH PLACED ON RIGHT HAND MAGNETO ONLY, ENGINE WOULD LOSE SUBSTANTIAL RPM. WITH ONLY LEFT MAGNETO SELECTED ENGINE WOULD RUN NORMALLY.