PLT OF A PIPER PA18 SUPERCUB MADE AN OFF ARPT EMER LNDG AFTER THE ENG QUIT DUE TO FUEL STARVATION. ONE OF THE CABIN SIDE WINDOWS CAME OFF INFLT EARLIER STRIKING THE L FUEL TANK DRAIN VALVE CAUSING A FUEL LEAK NOT NOTICED BY THE RPTR. THERE WAS NO DAMAGE TO THE ACFT.

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I WAS FLYING A PIPER SUPERCUB WITH THE SIDE WINDOW OPEN. FROM POSTFLT INSPECTION; IT APPEARS THAT 1/2 THE SLIDING SIDE WINDOW BLEW OFF THE AIRPLANE AND STRUCK THE L FUEL TANK DRAIN WHILE DEPARTING THE ACFT. I DID NOT NOTICE SINCE THE WINDOW WAS ALREADY OPEN. I REMEMBER THE WINDOW BEING PRESENT AT TKOF. THE L DRAIN STARTED LEAKING. BEFOREHAND I FLEW 10 MINS OFF THE L TANK TO VERIFY FUEL FLOW THEN FLEW 3 HRS ON THE R TANK UNTIL IT INDICATED 1/8. THE FUEL INDICATOR ON THE SUPERCUB IS A BALL IN A TUBE WHICH CAN BE IMPOSSIBLE TO SEE WHEN COMPLETELY FULL OR EMPTY. SINCE I COULD SEE NO BALL ON THE L INDICATOR AND I HAD VERIFIED THE TANK FULL ON THE GND; I BELIEVED IT TO BE FULL AND SWITCHED TO THE L TANK. ABOUT 1 MIN LATER THE ENG QUIT BUT RESTARTED UPON GOING TO THE R TANK. I STILL BELIEVED THE L TANK TO BE FULL AND PULLED SLIGHT POSITIVE AND NEGATIVE G'S TO SEE IF EITHER WOULD REVEAL THE BALL IN THE L TANK INDICATOR. THE ONLY EFFECT WAS THAT THE BALL FOR THE R TANK 'UNSTUCK' AND WENT TO EMPTY. I TURNED STRAIGHT FOR THE ARPT AND CLBED TO 4500 FT FROM 3500 FT. THE ENG QUIT AGAIN BEFORE REACHING THE ARPT AND I LANDED IN A FIELD ABOUT 1 MI SHORT OF THE RWY. I BELIEVE THE COMBINATION OF LEAKING L TANK; MISLEADING L FUEL GAUGE; AND STUCK R FUEL GAUGE WERE BEYOND MY CTL. THE OFF-FIELD LNDG ITSELF WAS UNEVENTFUL.