What happened
An initial takeoff attempt was delayed by fog. After completing a preflight inspection and refueling the aircraft with 11 gallons of aviation fuel, the pilot waited for visibility to improve. Once conditions cleared, the pilot performed a runup and began the takeoff roll. However, the pilot aborted this first attempt due to engine problems.
Following the aborted takeoff, the pilot conducted another runup procedure, which included testing the carburetor heat system, in an effort to correct the rough engine condition. A second takeoff attempt was initiated. As the aircraft climbed through 200 feet AGL, the engine developed a rough condition once again. The pilot attempted to restore full power, including the application of carburetor heat, but ultimately performed an emergency landing in a nearby river. There were no injuries reported.
The investigation
An examination of the aircraft and the engine assembly was conducted, but investigators failed to find any mechanical problems or subsystem malfunctions. The pilot reported that there had been no previous engine issues.