During the takeoff roll all engine indications were normal. However, shortly after liftoff the pilot noted that engine RPM was decreasing. Efforts to restore power were not successful, and the pilot was committed to a forced landing on rough terrain beyond the end of the flight strip. During the landing roll the left main landing gear was dislodged and a fuel tank was ruptured. Fire broke out and destroyed the aircraft. The subsequent examination of the wreckage was hampered because of the extensive fire damage. The only abnormality discovered was that those spark plugs which could be removed from the engine showed signs of being exposed to an over-rich fuel/air mixture. The fire damage to the relevant engine systems prevented the reason for this condition from being positively determined.