The pilot was carrying out a flight around the Archerfield Training Area. Approaching the southern boundary of the area he heard a change in engine note. The tachometer confirmed a loss of some 200 RPM, however, all other engine instruments indicated normal operating conditions. Full throttle selection failed to produce any change in engine RPM. The pilot elected to divert to Southport for a precautionary landing. After joining the downwind leg of the circuit he found that he was unable to control the engine RPM with the throttle. As a result he was unable to slow the aircraft to the normal landing speed. He decided against shutting the engine down and making a glide approach, as he was concerned that if he misjudged the approach the aircraft could land in the built up areas north of the aerodrome. On downwind once again, the RPM decreased to a level which was too low to sustain level flight. The pilot chose the clearest area in front of him for a forced landing. The area was a swamp and the aircraft nosed over shortly after touchdown. The engine had been overhauled and re-installed in the aircraft, 78 (in service) hours before the accident. The throttle arm linkage had come loose at the carburettor, effectively denying the pilot control of the engine. Examination of the castellated nut, which held the linkage in position, found no evidence that it had been torqued correctly or that a split pin had been installed.
