THE 107 HR PVT PLT & PSGR DEPD THE ARPT FOR AN UNKNOWN DESTN. THE RECENTLY PURCHASED ACFT HADN'T FLOWN FOR SVRL MONTHS. THE ACFT HAD BEEN PARKED OUTSIDE & 5.5' OF RAIN HAD FALLEN IN THE LAST 41 DAYS. THE ACFT WAS REFUELED IMMEDIATELY BFR DEPG & THE FUELER SAID THE PLT SAT INSIDE THE ACFT DRG REFUELING, READING A MAP. THE REFUELER DIDN'T SEE THE PLT MAKE A PREFLT INSPN. HE ALSO NOTED THE ACFT WAS VERY DIRTY. THE FUEL FILLER CAPS WERE OF THE THERMOS CAP STYLE, RECESSED INTO WELLS IN THE WING SFC. FAA SDR'S NOTED 20 INSTANCES OF WATER CONTAMINATION OF FUEL IN THIS MODEL ACFT DRG A PERIOD FROM 1/1/86 TO 9/10/92. OF THE 20 RPRTD INSTANCES, 4 WERE ASSOCIATED WITH LEAKAGE THRU LOOSE FUEL CAPS. DRG THE TAKEOFF & INITIAL CLIMB, THE PLT STATED TO THE TWR THAT 'MY ENGINE KEEPS CUTTING IN AND OUT.' HE WAS CLRD TO LND ON ANY RWY. THE PLT COMPLETED A 180 DEG TURN BACK TWD THE RWY & THE ACFT HIT A PWR POLE OFF THE END & BELOW THE RWY SFC. THE ACFT WAS CONSUMED BY A POST-CRASH FIRE. NO MECH MALFUNCTION/FAILURE WAS FND. SUNSET WAS AT 1712 PST.