At about 0945 Eastern time the airplane crashed off the end of the runway at Crystal River, Fla. Visual conditions prevailed and an instrument flight rules flight plan was filed. The Private pilot and three passengers were uninjured, although the airplane was substantially damaged. The flight had originated in Sarasota, Fla., about 45 minutes earlier. While in level cruise flight at 8000 feet, the engine started running roughly. The pilot was unable to maintain altitude, declared an emergency with the Jacksonville Air Route Traffic Control Center, and was given vectors for an airport at Crystal River, Fla. The pilot later stated that he flew the subsequent approach to a forced landing with too much speed and was unable to stop the airplane on the runway. During an attempted go-around, he avoided a fence but collided with trees. The pilot further stated that at no time did he attempt to use the airplanes carburetor heat control and added that, based on what occurred, he believes that he encountered carburetor icing.