DIVERT LAND DUE TO A FUEL PROB FUEL UNAVAILABLE DUE TO A VALVE FAILURE.

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I WAS USING FUEL FROM AUX FUEL TANKS WHICH SUPPLY APPROX 1 HR OF FLT TIME. AFTER USING THE AUX FUEL SUPPLY FOR 50 MINS; I ATTEMPTED TO RETURN THE FUEL SELECTOR VALVE TO THE 'MAIN' POS (MAIN FUEL TANKS; FRONT ENG). WHILE ATTEMPTING TO DO THIS; I COULD NOT CAUSE THE FUEL SELECTOR VALVE TO STAY IN THE 'MAIN' POS. I MADE 3 ATTEMPTS TO NO AVAIL; THE FUEL SELECTOR VALVE REMAINED IN BTWN THE 'MAIN' AND 'AUX' POS; NO DETENTS COULD BE FELT; AND THE ENG CONTINUED TO RUN. AS A PRECAUTION; I ABORTED THE SEARCH AND RESCUE MISSION; BEGAN TO CLB; AND BEGAN HDG DIRECTLY TO EYW. WHEN I REACHED 20 MI S OF KEY WEST; I CONTACTED KEY WEST APCH AND REQUESTED FLT FOLLOWING TO OPF. BOTH ENGS RAN WELL UP TO APPROX 20 MINS AFTER THE FUEL SELECTOR PROB WAS FIRST IDENTED. AT THAT POINT; ONE OF THE ENGS BEGAN TO LOSE PWR. I COULD NOT DETERMINE WHICH ONE BECAUSE BOTH ENGS SHOWED GOOD INDICATIONS; HOWEVER; I ASSUMED THAT THE FRONT ENG HAD SUFFERED FROM PARTIAL FUEL STARVATION. AT THIS POINT; I ADVISED KEY WEST APCH THAT I HAD AN 'URGENT' SIT; AND THAT I WISHED TO LAND AT EYW. HE ACKNOWLEDGED AND TOLD ME TO CONTACT EYW TWR. I DID SO; AND WAS CLRED TO LAND FROM 7500 FT MSL AND APPROX 1 MI SW OF EYW. DURING MY APCH TO RWY 9; A CTLR ASKED ME TO CONFIRM WHETHER I HAD 1 OR 2 OPERATIVE ENGS; AND I ADVISED HIM THAT I HAD 1 ENG PRODUCING PARTIAL PWR; AND 1 FULLY OPERATIVE ENG. THEY ALSO ADVISED ME 'EQUIP' WAS STANDING BY. I DID NOT REQUEST THE PRESENCE OF 'EQUIP;' BUT WAS GLAD THEY WERE THERE. DURING THE LNDG; THE FRONT ENG BECAME FULLY INOPERATIVE. AFTER THE AIRPLANE WAS PARKED; A FIRE MARSHALL ASKED ME HOW MUCH FUEL I HAD REMAINING IN THE TANKS. I TOLD HIM I DIDN'T KNOW BUT I ESTIMATED 80 GALS. HE SAID THIS INFO 'WASN'T FOR ANYTHING.' CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR FLIES THE CESSNA 337 ON SEARCH AND RESCUE MISSIONS FOR A CHARITABLE ORGANIZATION LOOKING FOR CUBAN 'BOAT PEOPLE.' THE ACFT WAS FACTORY EQUIPPED WITH AN AUX FUEL TANK FOR ADDED RANGE. THE SELECTOR VALVE WAS STUCK 'IN BTWN' AND; WHEN ATTEMPTING TO SELECT THE MAIN TANK; THE ASSEMBLY WAS BENT SO THAT IT COULD NOT BE OPERATED. THE VALVE HAS BEEN CHANGED AND THE SYS IS NOW WORKING PROPERLY.