On April 29, 2001, about 1831 eastern daylight time, a Piper J3-C65, N78521, was substantially damaged during a forced landing to a field in Clarksburg, West Virginia. The certificated commercial pilot was seriously injured and the passenger was not injured. Visual meteorological conditions prevailed and no flight plan was filed for the personal flight conducted under 14 CFR Part 91. According to the pilot, the flight originated at the Benedum Airport (CKB), Clarksburg, West Virginia. Before departure, the pilot performed a preflight inspection, set the fuel selector on the right fuel tank, and noted both fuel tanks were full of fuel, about 12 gallons total. He departed the airport and flew to a local farm where he performed several takeoffs and landings, giving rides to different passengers. The pilot stated that he flew for about 1 hour and 20 minutes on the right fuel tank until the right fuel tank gauge displayed 1/4 tank. He then switched to the left fuel tank, and started a timer to monitor his fuel consumption. The pilot conducted several more rides with passengers. The pilot reported that he picked up the last passenger, noted the time on the timer as 51 minutes, and flew to another farm where he intended to land. About 10 minutes later, he arrived at the farm and observed that the left fuel tank gauge read 1/4 tank. He performed a low approach to observe the field, and then initiated a go-around. During the climb, the pilot initiated a right turn to set up for a landing at the field, and the "engine quit." He leveled the wings, and performed a forced landing in the field beneath him. Examination of the airplane by a Federal Aviation Administration (FAA) inspector revealed substantial damage to the underside of the fuselage, the engine cowling and the landing gear. Both fuel tanks were intact, the left fuel tank contained 1 gallon of fuel and the right fuel tank contained about 6 gallons of fuel. The fuel selector was observed in the "off" position, and the throttle was noted in the "closed" position. The fuel system lines were blown through and verified to be clear with the fuel selector in the left and right positions. The engine was rotated by hand at the propeller flange and continuity was established through the valve train and powertrain to the accessory section. Compression on each cylinder was confirmed using the thumb method. Rotation of the propeller produced spark on all leads of both magnetos. All spark plugs were removed; their electrodes were intact and light gray in color. The carburetor was found separated from the engine. Examination of the carburetor revealed no evidence of fuel, and the inlet screen was absent of debris. The mixture control arm was noted in the full rich position. Examination of the airplane's logbooks revealed the last annual inspection was performed on October 25, 2000. The pilot reported about 1,495 hours of total flight experience, of which 375 hours were in make and model.