PLT OF A C340 RETURNED TO LAND DURING INITIAL CLB DUE TO BOTH ENGS BACKFIRING. FOUND PROB CAUSED BY THE ACFT SVCED WITH THE WRONG FUEL; JET A INSTEAD OF 100 OCTANE GASOLINE.

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I DEPARTED MEM AT APPROX XA30 WITH JUST THE TIP TANKS TOPPED FOR A 1 HR 10 MIN FLT TO SELMA; AL. ON ARR AT SELMA; THE WX WAS APPROX 7 MI VISIBILITY WITH SCATTERED CLOUDS AT ABOUT 500 FT; AND AN 1800 FT OVCST WITH RAIN. THERE WAS A MERLIN ON THE RAMP JUST IN FRONT OF THE FBO WITH THE ENGS RUNNING. I WAITED FOR HIM TO MOVE SO MY PAX WOULD NOT HAVE AS FAR TO WALK IN THE RAIN. I DO NOT KNOW IF HE GOT FUEL OR NOT. MY PAX DEPLANED AND WENT INSIDE THE FBO AS I WAS REORGANIZING THE INSIDE OF THE ACFT. AS I DEPLANED; THE LINEMAN APCHED ME AND SAID 'I UNDERSTAND YOU NEED SOME FUEL.' I REPLIED BY SAYING; 'YES; ONLY THE TIPS -- HUNDRED OCTANE -- I'M GOING IN TO CHK THE WX.' THE LINEMAN HAD FUELED THIS PARTICULAR ACFT BEFORE. NORMALLY; I STAND BY THE AIRPLANE AS IT IS BEING FUELED; BUT THE WX WAS RPTED TO BE 700-800 FT OVCST WITH RAIN IN MOBILE; AL; WITH THE POSSIBILITY OF ICE AND THE FSS KEPT ME ON THE PHONE FOR APPROX 15 MINS. BY THE TIME I GOT OFF THE PHONE WITH FSS; THE LINEMAN HAD ALREADY FINISHED FUELING THE ACFT. I NEVER EVEN SAW THE TRUCK HE USED TO FUEL THE ACFT. I DO NOT KNOW IF IT HAD THE FLARED NOZZLE TO PREVENT MISFUELING INSTALLED ON IT OR NOT. I GAVE HIM MY CREDIT CARD; WHICH HE RAN. HE PRESENTED ME WITH THE CREDIT CARD SLIP TO SIGN. IT ONLY HAD THE PRICE ON IT. I ASKED HIM HOW MANY GALLONS HE PUT IN. HIS REPLY WAS '48 GALLONS.' THE CREDIT CARD SLIP DID NOT INDICATE WHAT TYPE OF FUEL HE HAD USED OR HOW MANY GALLONS. I SIGNED IT. AT THAT TIME MY PAX WERE PROCEEDING BACK TO THE ACFT IN THE RAIN. I LET MY PAX IN; DID A WALKAROUND OF THE ACFT; CHKED THE FUEL CAPS AND SUMPED THE TANKS TO SEE IF ANY WATER WAS PRESENT. EVERYTHING APPEARED NORMAL. HOWEVER; I BELIEVE THERE WAS INSUFFICIENT TIME FOR ANY WATER TO SETTLE; OR THE JET FUEL TO MIX INTO THE SUMPS DUE TO THE SHORT TIME FROM THE FUELING. THE ACFT STARTED NORMALLY. I TAXIED OUT TO THE ACTIVE RWY AND PROCEEDED WITH MY PRETKOF CHKS AND STARTED MY TKOF. AS I RAN THE ENGS UP TO 27 INCHES OF MANIFOLD PRESSURE; I AGAIN CHKED ALL ENG GAUGES. THEY WERE NORMAL. AS I APCHED 80 KTS AND ROTATED; JUST AS I WAS AIRBORNE; ONE OR BOTH ENGS GAVE A BACKFIRE. I STARTED TO ABORT THE TKOF BUT DID NOT HAVE SUFFICIENT RWY AHEAD TO LAND. I WAS ABOUT 100 FT AND AGAIN CHKED ALL GAUGES. THE CYLINDER HEAD TEMPS STATED TO CLB TOWARD RED LINE AT THAT TIME AND THE ENGS STARTED TO RUN ROUGH. I REDUCED PWR; THE CYLINDER HEAD TEMPS STARTED TO REDUCE AND THE ENGS SMOOTHED OUT. I STAYED WITHIN GLIDING DISTANCE OF THE RWY; CIRCLED AND LANDED TO MAKE SURE EVERYTHING WAS NORMAL. AT THIS TIME I THOUGHT THERE MUST HAVE JUST BEEN A SMALL AMOUNT OF WATER CONTAMINATION DURING FUELING. I PULLED OFF THE SIDE OF THE RWY AND DID A COMPLETE ENG RUN-UP AGAIN. ALL GAUGES AND TEMPS WERE NORMAL. I AGAIN PROCEEDED TO TAKE OFF. THE ENGS RAN PERFECTLY. AS I ROTATED AND BECAME AIRBORNE; I CHKED THE ENG GAUGES AGAIN AND NOTED THE CYLINDER HEAD TEMPS WERE AGAIN APCHING RED LINE. I PULLED THE PWR BACK AND LANDED ON THE REMAINING RWY AND TAXIED BACK TO THE RAMP; TELLING THE PAX SOMETHING WAS WRONG AND WE COULD NOT PROCEED WITHOUT FINDING OUT WHAT IT WAS. AS I OPENED THE DOOR; THE LINEMAN APCHED THE ACFT. I WAS THINKING THAT THE FUEL HE PUT IN WAS OLD OR JUST CONTAMINATED AND ASKS; 'JUST WHAT KIND OF FUEL DID YOU PUT IN THIS?' HIS REPLY WAS 'JET A.' AT THAT POINT; I HAD THE LINEMAN DRAIN ALL THE FUEL FROM THE TIP TANKS. AS HE WAS DOING THAT; I CALLED THE SVC MGR FOR ADVICE ON HOW TO FIX THE PROB. AFTER DISCUSSING WHAT HAPPENED; WE GNDED THE ACFT AND DROVE BACK TO FAIRHOPE; AL.