What was found
THE CREW REPORTED FLUCTUATING IDLE DURING GROUND CHECK. THE CREW RETURNED TO THE RAMP. TYPICALLY THIS IS NOT CONSIDERED TO BE A REPORTABLE EVENT, HOWEVER THIS ENGINE/AIRCRAFT COMBINATION HAS BEEN THE SUBJECT OF AN AIRWORTHINESS DIRECTIVE REGARDING UNSTABLE IDLE/MIXTURE ISSUES. IT IS NOT UNCOMMON FOR THIS SITUATION TO OCCUR NOT ONLY DURING INSPECTION GROUND RUNS, BUT ALSO DURING OPERATION BETWEEN INSPECTIONS. A COMMON SOURCE OF PROBLEMS WITH THIS FUEL CONTROL IS THE MIXTURE VALVE, PN-2520626. THIS VALVE BECOMES SCORED AND THEN INACCURATE IN IT'S FUNCTION. THERE IS A PROCEDURE FOR LAPPING THIS VALVE'S SURFACE.