SMA PLT ON PHOTO MISSION HAS LOSS OF PWR DUE TO LOW FUEL. DIVERT LAND.

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I PREPARED FOR A VFR PHOTO FLT TO JAMESTOWN; NY; WITH AERIAL PHOTOGRAPHER. I OBTAINED MY FIRST WX BRIEFING FROM BUFFALO FSS. WX AT JAMESTOWN WAS IFR SO I DELAYED DEP UNTIL I COULD RECEIVE THE LATEST HRLY WX OBSERVATIONS. I OBTAINED AN UPDATED WX BRIEFING WHICH SHOWED JAMESTOWN TO BE 2000 FT BROKEN AND FORECAST TO IMPROVE. AT THIS POINT; I CONSIDERED FILING IFR TO JAMESTOWN; THEN OBTAINING A DSCNT TO VISUAL CONDITIONS. AT THIS POINT; I BELIEVED THAT WE WOULD BE USING OUR SMA Y. UPON VIEWING THE SCHEDULE; I SAW THIS WAS NOT SO. REALIZING THAT THE ACFT WE WERE SCHEDULED TO USE; AN SMA X; WAS NO LONGER INSPECTED FOR IFR; I DETERMINED THAT UNDER THE GIVEN CONDITIONS; IT WAS SAFE TO PROCEED UNDER VFR TO OUR DEST. I THEN PROCEEDED TO PREFLT FOR THE FLT. ALL SYS APPEARED TO BE IN WORKING ORDER. BOTH FUEL GAUGES READ FULL; AND UPON VISUAL INSPECTION OF THE ACFT FUEL TANKS; I FOUND THE R FUEL TANK TO BE FILLED TO OVERFLOWING. THE L FUEL TANK WAS DOWN APPROX 3/4 INCH FROM THE TOP. AT THIS POINT; I ESTIMATED THE TOTAL FUEL IN THE ACFT TO BE 23 OR 23.5 GALLONS. THIS WAS LATER FOUND TO BE IN ERROR. AFTER TAXI AND RUN-UP; WE DEPARTED SYR. THE PROPOSED RTE WAS TO FLY TO JAMESTOWN VIA THE GENESEO VOR; TO AVOID HIGHER TERRAIN TO THE S. 20 MINS OUT FROM SYR; NEAR GENEVA; NY; THE WX IMPROVED TO 3500 FT SCATTERED TO BROKEN. I CALLED THE BUFFALO FSS WITH THE ACFT RADIO ON 122.6. THIS RADIO CALL GAVE ME MORE INFO ABOUT THE IMPROVING WX AT JAMESTOWN; AND I FILED A PIREP ABOUT THE CLOUD BASES. WE ARRIVED AT THE PHOTO AREA AT OUR ETA. I NAVIGATED THE ACFT TO THE SW EDGE OF THE LAKE AT JAMESTOWN; APPROX 5 MI SW OF THE ARPT. THE PHOTOS TOOK APPROX 15 MINS; AND WE WERE READY TO DEPART. BASED ON THE WINDS ALOFT FORECAST AND MY OWN ESTIMATES OF THE WIND; I DETERMINED THAT WE HAD ENOUGH FUEL AT THIS POINT TO REACH SYR WITH A VFR FUEL RESERVE OF APPROX 40 MINS. WITH NO EXPECTED DELAYS; I DETERMINED THAT WE WOULD BE WELL WITHIN THE LIMITS OF THE ACFT'S FUEL ENDURANCE. NEAR JAMESTOWN; I CALLED BUFFALO FSS ON 122.1. THEIR WINDS ALOFT FORECAST INDICATED THAT I WOULD HAVE A 25 KT TAILWIND AT MY RETURN CRUISING ALT OF 3500 FT. ON 2 SEPARATE GNDSPD CHKS; I DETERMINED MY GNDSPD TO BE 135 KTS; THEN 130 KTS. OUR ETA WAS STILL WITHIN THE ENVELOPE I HAD DETERMINED TO BE SAFE. AS WE APCHED NEAR 40 MI FROM SYR; GNDSPD SEEMED TO BE SLOWING AND I CALLED UNICOM TO INFORM THEM THAT I EXPECTED TO BE ON THE RAMP AT XA10. A FEW MINS LATER; NEAR MARCELLUS; NY; I EXPERIENCED A SUDDEN LOSS OF PWR AND ENG ROUGHNESS. I SUSPECTED CARB ICE AND APPLIED CARB HEAT AND ENRICHED THE FUEL MIXTURE. WHEN THIS HAD NO APPARENT EFFECT; I PROCEEDED TO A LNDG AREA. WHILE SPIRALING OVER THE AREA WITH PARTIAL PWR; THE ENG CAME BACK TO FULL PWR. I HAD PREVIOUSLY INFORMED SYR APCH OF MY SIT. I THEN TOLD THEM I HAD FULL PWR. THEY GAVE ME A REF TO FIND MARCELLUS ARPT; AND I PROCEEDED THERE FOR A LNDG UNDER FULL PWR AND CTL OF THE ACFT. UPON LNDG; I TAXIED AWAY FROM THE RWY TO THE ARPT BUILDING. I CALLED MY FBO AND OUR CHIEF FLT INSTRUCTOR CAME TO ASSIST. UPON ARR; HE QUESTIONED ME AND INSPECTED THE ACFT. TOGETHER; WE DETERMINED THAT THE MOST LIKELY CAUSE OF THE PWR LOSS WAS INTERRUPTION OF THE FUEL FLOW TO THE ENG; CAUSED BY OP NEAR THE AIRPLANE'S UNUSABLE FUEL LIMIT.