What was found
DURING ENGINE OPERATION, THE MIXTURE CONTROL STOP PIN HAD BECOME LOOSE AND VIBRTED OUT PARTIALLY, LOCKING THE MIXTURE IN THE FULL RICH POSITION. IT IS SUSPECTED THAT DUE TO HEAVY USE AS A TRAINING ACFT WITH REPEATED STOP TO STOP OPERATION OF THE MIXTURE CONTROL AS WELL AS POSSIBLY THE METHOD OF SECURING THE PIN INTO THE FUEL PUMP HOUSING FROM THE MFG AND/OR O/H-REPAIR FACILITY ALLOWED FOR THE LOOSENING OF THE PIN IN THE HOUSING. TO PREVENT RECURRENCE IN THE FIELD, A CLOSER INSPECTION OF THE STOP PIN TO MIXTURE CONTROL ARM CLEARANCE ALONG WITH CHECKING THE STOP PIN FOR LOOSENESS AT REGULAR INTERVALS IS ADVISABLE. TO PREVENT RECURRENCE FROM THE MFG AND/OR O/H/REPAIR FACILITY, A MODIFICATION TO ALLOW FOR A ROLL PIN INSTALLATION TO SECURE THE PIN IN THE FUEL PUMP HSG MAY BE ADVISABLE.