The airplane was being flown on a local sightseeing flight. The pilot said he was at about 2,500 feet at cruise RPM. After the airplane crossed the dam, the RPM dropped to idle and the engine began to run rough. The pilot selected a dirt road as a potential landing site. As he turned final, the engine regained partial power. The pilot elected to perform a go around, at which time the engine again lost power. The pilot elected to land on a roadway. During the final approach the left wing struck the ground first, causing the airplane to cartwheel. The investigation revealed the primary and main venturi was a two piece design and the primary venturi became loose, which caused the loss of engine power. Two service bulletins were not performed which would have replaced the two piece design with a one piece venturi. The airplane received an annual inspection 3 months before the accident and had accrued 20 flight hours since the inspection.