On August 21, 2010, about 1210 eastern daylight time, a Piper J3C-65, N209RC, registered to a private individual, sustained substantial damage when it struck a parked a car during a forced landing shortly after takeoff from Southern Cross Airport, Williamstown, New Jersey. Visual meteorological conditions prevailed and no flight plan was filed for the personal flight conducted under the provisions of 14 Code of Federal Regulations (CFR) Part 91. The private pilot, who was the sole occupant, sustained minor injury. There were no occupants inside the car which was parked adjacent to a house. The flight was originating at the time of the accident. The pilot stated that since the airplane had not been flown recently, he conducted two preflight inspections, an engine run-up, and two full-power takeoff test runs becoming airborne briefly with no discrepancies noted. After the two test takeoff runs, the pilot taxied to the approach end of runway 27, and slowly advanced full throttle to takeoff. The airplane accelerated and became airborne at the same point along the runway it had during the two test runs. During the initial climbout while flying the runway heading, the pilot noticed that the engine was suddenly no longer developing full takeoff power and performed a quick scan of the cockpit to ensure that the fuel selector was on, the carburetor heat was off, and the throttle was full forward. As the airspeed began to decrease along with the altitude, the pilot was not sure if the airplane would clear powerlines at the departure end of the runway. Since he was too far down the runway to abort, he continued to fly, cleared the powerlines, and maneuvered the airplane for a forced landing in an adjacent field. The airplane then clipped the tops of trees, and impacted an unoccupied parked car. After impact, the pilot exited the airplane and waited for rescue services. Postaccident examination of the airplane and engine by an FAA inspector revealed that the fuel tank was nearly full of 100 low lead fuel and no contaminants were noted in the fuel tank or fuel strainer. The carburetor, which was separated at impact, was inspected and no discrepancies were noted. Examination of the engine revealed crankshaft, camshaft, and valve train continuity, and compression testing of all cylinders revealed no discrepancies. No ferrous particles were noted in the engine oil. Operational testing of the spark plugs was performed using 80 psi as a reference. A weak spark from both plugs of the No. 1 cylinder was noted during testing. The lower plugs of the remaining cylinders which were found oil soaked, and tested below the normal range. The FAA inspector also reported that the cylinders were last replaced in 2006, and at the time of the accident the engine had only accumulated approximately 6 hours since the cylinders were replaced.