A PA25 ENG FAILED DURING A TANK SWITCH IN THE DSCNT. AFTER INVESTIGATION BY A MECH THE CAUSE WAS NOT DETERMINED OR REPEATED SO THE RPTR DEPARTED FOR A FERRY FLT TO BRAZIL. ON THE FIRST LEG; OVER WATER; THE ENG BEGAN TO RUN ROUGH AND THE RPTR HAD TO MAKE AN EMER LNDG AT CHUB CAY BAHAMAS. ON THE GND; WHEN THE RPTR INSPECTED THE FUEL SYS; HE FOUND SILICON DEBRIS BLOCKING THE FUEL FLOW WHICH HAD CAUSED THE ENG PROBS.

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ACFT WAS RETURNING TO FXE IN PREPARATION FOR FERRY FLT TO BRAZIL. DURING APCH TO FXE PRIOR TO OVERFLT OF POPULATED AREA I BEGAN LNDG CHKLIST WHICH INCLUDED A CHANGE OF FUEL TANKS. DURING WHICH THE ENG BEGAN TO QUIT; THEN UPON RETURNING TO AUX TANK THE ENG DID NOT RESPOND AND AN EMER LNDG WAS INITIATED AND COMPLETED WITH NO INJURIES OR ACFT DAMAGE. THE FOLLOWING DAY AFTER I HAD AN A&P MECH INSPECT THE ACFT AND AFTER A DIFFICULT TIME GETTING ACFT STARTED; WE DID RUN-UPS AND IT WAS DETERMINED THAT THERE WAS NO PROB. TKOF WAS UNEVENTFUL AND FLT TO FXE AND A SUBSEQUENT INSPECTION DID NOT REVEAL THE PROB AND I CONTINUED ON THE FERRY MISSION. I DEPARTED FOR EXUMA BAHAMAS ON THE FIRST LEG. AFTER PASSING BIMINI I NOTICED ENG ROUGHNESS. ALTHOUGH AT FIRST I PASSED THIS OFF AS AUTOMATIC ROUGH OVER WATER; I SOON RECOGNIZED A REAL PROB AND NOTIFIED NASSAU RADIO THAT I WOULD BE MAKING AN EMER LNDG AT CHUB CAY BAHAMAS WITH ENG PROBS. AFTER INSPECTING THE SYS I FOUND THAT YOU COULD SEE A LARGE AMOUNT OF DEBRIS IN THE STARBOARD SIDE OF THE HOPPER GATE. AFTER FASHIONING A HOOK I RETRIEVED SEVERAL LARGE PIECES OF SILICON. CONCLUSION: FUEL HAD DETERIORATED AND RESTR THE FUEL FLOW CAUSING THE ENG PROBS. NOTE: THIS IS NOT THE FIRST TIME I HAVE EXPERIENCED THIS PROB AND THIS COMPANY KNOWS FULL WELL OF THIS PROB. I HAVE SINCE BEEN INFORMED OF 2 OTHER AG ACFT IN 1995 THAT WENT INTO THE SEA DURING A TANK CHANGE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE SILICON THAT WAS FOUND IN THE HOPPER GATE WAS ORIGINALLY USED TO SEAL THE CHEMICAL HOPPER TANK TO PREVENT LEAKAGE DURING CROP DUSTING OPS IN THIS PA25. AFTER THE TANK HAD BEEN MODIFIED TO CARRY FUEL FOR FERRYING; THE SILICON BEGAN TO DETERIORATE. THE RPTR BELIEVES THAT THE INTRODUCTION OF TEMP EXTREMES PROMOTED THE PROB. IT TOOK VARIOUS EXPERIENCES WITH ENG FAILURES FOR THE RPTR TO MAKE THIS CONNECTION; BUT RPTR BELIEVES THAT THE SILICON IS THE PROB AND STATES THAT A FRIEND OF HIS RECENTLY HAD THE SAME THING HAPPEN TO HIM. THE MAIN CONCERN AND REASON FOR FILING THIS RPT WAS SO THAT OTHERS WOULD NOT EXPERIENCE THE SAME PROB. THIS PLT STATES THAT THE PERMISSION TO MODIFY AND OPERATE WITH THE FAA FORM 337 ONLY REQUIRES A CONNECTION; FILTER; AND VALVE; AND A LEAK TEST AND VISUAL INSPECTION OF THE TANK.