The pilot departed on a 42 nautical mile cross country flight in his homebuilt single place experimental aircraft and was in cruise flight when he encountered turbulence and the engine quit. The pilot reported that he encountered rough air at his cruise altitude of 4000 feet and decided to descend to 3800 feet for smoother air. As he began the descent the engine began to run rough and then quit. During the forced landing the airplane struck a fence and mesquite trees. The airplane sustained substantial damage to the wings, landing gear, and empannage. A Federal Aviation Administration Inspector, during an on scene investigation, found the carburetor had separated from the intake manifold at the flexible rubber couplings.