The aircraft had previously suffered from aileron flutter and a new set of wings had been constructed and installed. Although the problem of flutter had been removed the aircraft then flew slightly right wing low. A trim tab was fitted and the subject flight undertaken to check its setting. Initial engine running and flight after take-off were normal until the aircraft was on the downwind leg of the circuit. The engine speed was heard to reduce and the aircraft settled into a glide descent. The left wing struck a tree and the aircraft landed heavily on its left wheel and wing tip. The wing separated at its root and the aircraft bounced, landing again on its left side 25 metres further on. Specialist examination of the aircraft indicated that the fuel primer bulb in the engine fuel line had a valve installed incorrectly. Under certain orientation conditions the valve could prevent an adequate fuel flow for continued engine operation. The fuel system layout was such that the position of the primer bulb could vary after handling between flights. As the engine was either stopped or running at idle power at the time of ground impact, the fuel flow is believed to have been restricted, causing engine fuel starvation. No other engine or airframe defects were found. Calculations showed that flutter would not have occurred in the re-designed ailerons and no evidence of flutter was found in the wreckage. When faced with a forced landing in moderately treed terrain, the pilot had misjudged his distance from a tree and was unable to regain control of the aircraft in the height available.