Approaching the destination, the engine began to run roughly. As the aircraft was losing altitude the pilot selected a track running through the scrub as the only suitable landing area. During the landing roll both wings struck trees and the aircraft ran off the track. Rough running was caused by a worn carburettor float valve assembly, which in turn stuck closed and then too far open. Consequently the engine first ran too lean, and then too rich. Once rough running began the pilot selected the mixture to full rich. This did not resolve the problem, but the pilot left the control in full rich instead of attempting to adjust the mixture through the full range available.