On March 25, 2002, approximately 0900 mountain standard time, an experimental Potts KitFox, N139SP, impacted the waters of the Salmon River during a takeoff attempt from a private airstrip near Shoup, Idaho. The commercial pilot, who was the sole occupant, was not injured, but the aircraft, which was owned and operated by the pilot, sustained substantial damage. The Title 14, CFR Part 91 personal pleasure flight, which was being operated in visual meteorological conditions, was departing for Challis, Idaho, at the time of the accident. No flight plan had been filed. According to the pilot, who was departing his 1,000 foot private strip, about the time the aircraft got half way down the runway, he realized the engine was not producing as much power as he expected. He momentarily thought about aborting the takeoff, but instead elected to continue. Although he was able to get the airplane in the air, he was unable to get it to climb. After crossing the departure end of the runway, and only being able to keep the aircraft a few feet off the ground, he attempted to make a shallow turn in order to fly down the Salmon River. During the turn, he was unable to maintain sufficient airspeed, and the aircraft began to stall/mush. As it started to drop, one wing made contact with the river. Immediately after the wing drug in the water, the aircraft cartwheeled along the river's surface, and it came to rest upright in about 30 inches of water. According to the pilot, when he changed the experimental aircraft's powerplant to a new Franklin engine from a Rotax engine, he did not change the propeller. Although he had shortened the blades of the propeller approximately one inch, and sent the carburetor to the factory for inspection and service, he was never able to get the engine to run at the expected 2,800 rpm. Although the engine would not reach the expected rpm, the pilot reported that it seemed to run smooth and steady. Just prior to the subject takeoff, the pilot did a magneto check at 1,800 rpm, and as he selected each magneto individually, the engine slowed by 250 rpm.