On February 17, 1994, at 0930 eastern standard time, a Piper PA-28-140, N98321, was substantially damaged during a forced landing in Stockbridge, Georgia. The airline transport pilot/flight instructor and his student had minor injuries. The aircraft was owned and operated under 14 CFR Part 91 by the student pilot. Visual meteorological conditions existed at the time, and no flight plan was filed for the local, instructional flight. The flight was originating at the time of the accident. The flight instructor reported the following: During the initial climb after takeoff, the engine began to "miss" and lose power. An application of carburetor heat did not correct the problem. The engine began to develop more power, and a climb was attempted. The engine lost power again, and the altitude could not be maintained. A turn toward an open field was made. A sharp bank angle was required because of trees in the area. He rolled out, and flared the airplane. The aircraft landed hard in the field, resulting in structural damage to the airframe. A Federal Aviation Administration (FAA) airworthiness inspector examined the aircraft following the accident. He reported the following: The carburetor bowl was full of clean, automotive fuel. The spin-on oil filter was cut open; it contained clean oil with no contaminants. All engine controls had full continuity. The ignition switch and "p" leads were functionally tested and operated normally. Both magnetos sparked all ignition leads when rotated. The inspector noted that the oval dry paper air filter, Piper Part Number 460632, showed evidence of being recently water soaked. The accident pilot and the aircraft owner reported that the aircraft had been washed with water just prior to the takeoff for the accident flight. For a more detailed description of the wreckage examination, refer to the FAA Inpsectors' statements, attached to this report.