On September 14, 2005, at 1835 central daylight time, a Ryan Navion G, N2401T, sustained substantial damage when it impacted a fence and terrain during a forced landing after a loss of power during takeoff from runway 4 (2,300 feet by 40 feet, asphalt) at Lake Geneva Aire Estates Airport (WI89), Lake Geneva, Wisconsin. The pilot was not injured. The 14 Code of Federal Regulations Part 91 personal flight was departing with Burlington Municipal Airport (BUU), Burlington, Wisconsin, as the destination. Visual meteorological conditions prevailed at the time of the accident. No flight plan was filed. The pilot reported that he took fuel samples from four points on the belly of the airplane during the preflight and did not find any water contamination. He reported that during the taxi for takeoff, the engine almost quit so he placed the mixture to full rich and the engine ran smoothly. He reported that he completed the engine run-up and taxied into takeoff position and "everything looked good." He reported that he put in full power, held the brakes for five seconds, and then released the brakes. The airplane lifted off at 80 mph and started to climb. The pilot reported, "At about 40 ft agl the engine went flat. I checked airspeed and I was at 60 mph so I put the nose down." The airplane hit a fence at the end of the runway. The pilot reported that the engine regained full power and the airplane impacted the terrain and went across the street and into a field before he shut down the engine. The pilot exited the airplane without injury. A Federal Aviation Administration airworthiness inspector examined the airplane. The inspection of the airplane revealed no deficiencies that would have precluded normal engine operation. The pilot reported that he had not flown the airplane in the previous two weeks and that he parked the airplane on a slight hill during that time rather than in a hangar. He reported that the hill was not level and that it sloped down and to the right. He reported that as a result of the sloped hill, the airplane's wing fuel tanks were not level when he checked for water contamination during the preflight. He reported that it rained the night before the accident flight, and that he found water around the wing's filler caps.