A VELOCITY XLRG ON A TEST FLT EXPERIENCED AN ENG FAILURE AND LANDED OFF ARPT.

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ALL CHKLIST ITEMS FOR PREFLT; START; TAXI AND BEFORE TKOF COMPLETED PER FLT MANUAL PRIOR TO FLT. I DEPARTED ZZZ1 FOR PHASE I FLT TESTING. 5 HRS OF FLT TESTING WERE PREVIOUSLY FLOWN. I PROCEEDED 8-10 MI E OF ZZZ1 AT 3500 FT MSL. AFTER ABOUT 15 MINS OF TURNS; CLBS; AND DSCNTS; THE ENG BEGAN TO LOSE PWR. RETURN TO ZZZ1 INITIATED AND EMER PROCS ACCOMPLISHED. ZZZ1 TWR WAS NOTIFIED OF ENG PROB AND A CLRNC FOR A DIRECT ENTRY FOR LNDG WAS RECEIVED FROM TWR. TWR WAS NOTIFIED THAT THE ENG HAD STOPPED AND A LNDG IN A NEARBY FIELD WOULD BE MADE. THE LNDG WAS COMPLETED SUCCESSFULLY. THERE WERE NO PERSONAL INJURIES AND DAMAGE TO THE ACFT WAS RESTR TO THE GEAR DOORS. THE FIELD WAS DESERTED; NO NEARBY HOMES OR BUILDINGS AND NO PROPERTY DAMAGE. ACFT WAS REMOVED FROM FIELD BY TOW TRUCK AND RETURNED TO ZZZ1 HANGAR. CAUSE OF PWR LOSS YET UNDETERMINED. FUEL DELIVERY SYS BEING EXAMINED. PRECEDING THE FLT; THE CYLINDER HEADS WERE REMOVED; RECONDITIONED; AND REPLACED. THIS REQUIRED THE REMOVAL OF THE INTAKE MANIFOLD FUEL INJECTION SYS. ALL FUEL FILTERS WERE CLEANED AND THE INTAKE MANIFOLD AND FUEL LINES CONNECTED. ALL REPAIR WORK WAS DONE ACCORDING TO MANUFACTURER'S SPECS. THE ENG WAS RUN TESTED FOR 15-20 MINS; CHKED FOR LEAKS. ALL FUEL LINES INSPECTED AND ENG CTLS CHKED. INTAKE MANIFOLD BOLTS RE-TORQUED. ACFT THEN TAXIED AND ENG CHKED AT PWR SETTINGS UP TO AND INCLUDING TKOF PWR. TOTAL ENG RUN TIME AFTER MAINT WAS 45-60 MINS. ALL SYS CHKED AND ACFT WAS RETURNED TO SVC. CAUSE OF PWR LOSS STILL UNDETERMINED WITH FUEL DELIVERY SYS UNDER EXAMINATION. THE ACFT HAD FLOWN 5 HRS PREVIOUSLY WITH NO HISTORY OF FUEL SYS PROBS.