During a takeoff attempt from a 1,500-foot turf airstrip, towards rising terrain, a Piper PA-28-161 impacted trees. According to the pilot, during the preflight run-up, when the right magneto was selected, the engine dropped 500 rpm. He attempted to "clean up the plugs," but the engine still "dropped" more rpm than when the left magneto was selected. He selected "one notch of flaps" for takeoff. During the takeoff roll, the airplane did not seem to accelerate as expected. The pilot then selected a "second notch" of flaps at approximately 35 knots indicated airspeed. At midfield, the airplane was still not airborne and the pilot became concerned about not having enough remaining runway; however, he decided to continue the takeoff. The airplane became airborne, but would not climb. The stall warning "went off," and the airplane struck trees. The pilot added that the engine rpm always dropped more when the right magneto was selected rather than the left. A review of the airplane's documentation by a Federal Aviation Administration inspector revealed that no weight and balance information was in the airplane. A review of the accident airplane's information manual and original delivery documents revealed that under the conditions that existed at the time of the accident, the takeoff distance required to clear a 50-foot obstacle was approximately 1,500 feet and was based on the use of a "paved, level, dry runway."