On August 31, 2001, at 1400 hours Pacific daylight time, a Funk C, N22694, was substantially damaged when it landed in a field following a loss of engine power on takeoff from Merced, California. The private pilot, who was the sole occupant, was not injured. Visual meteorological conditions prevailed for the flight operated by the pilot under 14 CFR Part 91. The flight was originating to return to the home field, having flown to Merced for lunch. The pilot stated that he had departed the Merced airport approximately 1400. He climbed upwind until reaching an indicated altitude of 700 feet, and then began a left turn to 210 degrees for an on-course heading to Frazier Lake. At this point, he began to feel a slight loss of engine power, followed by a slow reduction of engine power, which eventually reached a nearly wind milling condition. He pumped the throttle, but the engine did not respond. He attempted to return to the airport, but realized he was too far away to land on the runway. He landed in a plowed field, where the airplane nosed over. According to the pilot, the wing tanks were empty at the time of departure, and the only tank in use was the fuselage tank. He stated that a possible cause of the engine failure was the use of a nonvented fuel tank cap. A post accident examination of the engine after the airplane was recovered to a maintenance facility indicated that the engine did not appear to be damaged. The engine compression was within the manufacturer's acceptable tolerances. The ignition system had no defects noted. The carburetor, throttle, and mixture controls inspection revealed that Airworthiness Directive 98-01-06 had been complied with, and a one piece venturi was installed in the carburetor. The drain plug was removed from the carburetor fuel bowl and fuel drained from the fuselage tank. It was noted that the fuel slowed down after approximately 2.5 gallons. The fuselage fuel tank cap was then removed with a vacuum sound heard while loosening the cap. After removing the cap, the fuel flowed freely again. The fuselage fuel tank was not vented, and a vented fuel cap was not installed. The pilot stated in his report that he had switched fuel caps prior to the last annual inspection that occurred 8 flight hours and 6 months prior to this flight, and did not record the change in the aircraft records.