THE PILOT OF THE PIPER J-5 HAD JUST PURCHASED THE AIRCRAFT AND WAS RETURNING TO HIS HOME AIRPORT WHEN THE AIRCRAFT RAN OUT OF FUEL. DURING THE ENSUING EMERGENCY LANDING THE AIRCRAFT HIT A DITCH AND NOSED OVER. ACCORDING TO THE PILOT THE AIRCRAFT HAD BEEN BURNING ABOUT EIGHT GALLONS OF FUEL PER HOUR. THE AIRCRAFT, WHICH HAD BEEN AIRBORNE APPROXIMATELY THREE AND ONE HALF HOURS, SHOULD HAVE HAD AN ENDURANCE OF OVER FOUR HOURS AT CRUISE. AFTER THE ACCIDENT IT WAS DISCOVERED THAT THE LEFT FUEL TANK CAP HAD BEEN PUT ON WITH THE PRESSURIZATION TUBE FACING AFT INSTEAD OF FORWARD INTO THE SLIPSTREAM. BOTH THE PILOT AND THE FAA INSPECTOR WHO RESPONDED TO THE ACCIDENT FELT THAT THE BACKWARDS CAP HAD SIPHONED OFF SOME OF THE FUEL, LEADING TO FUEL EXHAUSTION. THERE WAS NO EVIDENCE OF AN ENGINE MALFUNCTION OR OTHER SOURCE OF FUEL LEAKAGE.