On May 16, 2001, at 1940 central daylight time, a Piper J5A, N38646, operated by a commercial pilot, sustained substantial damaged when it nosed over during the landing roll following a precautionary landing. The pilot and passenger were not injured. The 14 CFR Part 91 personal flight had departed St. Charles (3SQ), Missouri, at 1930 on a local flight. The precautionary landing was made to an unnamed grass airstrip in the Two Rivers National Wildlife Area near Brussels, Illinois. Visual meteorological conditions prevailed and no flight plan was filed. The pilot reported he was practicing a simulated emergency landing approach. He reported he had pulled out the carburetor heat during the simulated emergency landing and cleared the engine during the approach. He terminated the maneuver at 300 feet above ground level (agl). He reported that when he advanced the throttle there was no response for a "short period of time, then a halting or missing response." He reported the engine made a "hesitating acceleration." The airplane was about 200 feet agl and he made a decision to land and check the fuel and engine. The airplane touched down at 65 mph. He reported the vegetation was much taller than he perceived and the airplane decelerated rapidly and nosed over. The engine was examined and exhibited no preexisting anomalies. The engine was tested and it ran between idle power and 2,600 rpm.