The airplane had just been purchased and was being ferried to a new location. The pilot said he had leveled off at 3,500 feet, and was attempting to lean the fuel mixture, but the engine started running rough. He placed the mixture control back to full rich and the engine smoothed out, so he decided to fly the remainder of the flight in full rich. He said the engine ran good for five minutes, then the RPM started to fall from 2,400 to 1,500, and he was unable to maintain altitude and airspeed. ATC furnished vectors toward the nearest airport, but the engine quit completely and the pilot had to make a forced landing in a heavily wooded area. Post accident investigation revealed that the mixture control housing, at the carburetor, had slipped from the attachment clamp. The mixture control arm was found in cutoff and it was found that it would not move out of cutoff, despite the position of the mixture control in the cockpit. It could be moved manually, at the carburetor. The aircraft had an annual inspection 1.6 hours before the accident.