C172 PLT HAS FUEL STARVATION AND LANDS OFF FIELD.

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I HAD BEEN FLYING FOR APPROX 1 HR. I WAS IN STRAIGHT AND LEVEL CRUISE FLT AT ABOUT 1700 FT MSL WHEN THE ENG BEGAN TO RUN ROUGHLY; AND THEN THERE WAS A LOSS OF MOST OF THE ENG PWR. I DETERMINED THAT I WAS TOO FAR AWAY FROM THE NEAREST ARPT (TKI) TO SAFELY MAKE IT; SO I CHOSE A SUITABLE PASTURE FOR AN OFF-ARPT LNDG. THE PASTURE WAS A FAIRLY LEVEL VACANT FARM FIELD IN A RURAL AREA; PERHAPS 100 ACRES OR SO. I LANDED IN THE FIELD WITHOUT CAUSING ANY DAMAGE TO THE PROPERTY. MY AIRPLANE HAD SOME SCRATCHES ON THE PROP FROM GND BRUSH/WEEDS. I LEFT THE AIRPLANE IN THE PASTURE; AND RETURNED THE NEXT DAY WITH AN A&P TO FIND THE CAUSE OF THE ENG ROUGHNESS. THE A&P DETERMINED THAT IT WAS A BLOCKED GAS TANK FUEL VENT LINE; THAT WAS CAUSING GAS STARVATION TO THE ENG. THE VENT LINE BLOCKAGE WAS CLRED AND THE AIRPLANE RAN FINE AGAIN. THE OWNER OF THE PASTURE WAS CONTACTED AND GAVE PERMISSION FOR ME TO FLY THE AIRPLANE OUT. THE PASTURE HAD ADEQUATE LENGTH AND THERE WASN'T ANY PROB IN THE 'GRASS RWY' TKOF. I FLEW THE AIRPLANE BACK TO MY HOME ARPT WITHOUT ANY PROBS. MY AIRPLANE HAS A VENTED FUEL TANK CAP ON 1 GAS TANK. THE A&P SUGGESTED THAT A GOOD PREVENTION MEASURE WOULD BE TO HAVE VENTED FUEL TANK CAPS INSTALLED ON BOTH TANKS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED HE HAD BOTH FUEL TANKS SELECTED WITH THE FUEL SELECTOR VALVE. HE DID NOT NOTICE HIS FUEL QUANTITY DIFFERENCE BTWN THE 2 TANKS.