Circumstances:
The pilot had planned to carry out a series of circuits and landings. During the downwind leg of the first circuit, the engine began to run roughly and then lost power. The pilot attempted a forced landing on a clearing adjacent to the airstrip boundary fence. During the landing roll the pilot lost directional control due to the rough surface and the aircraft collided with the fence. Subsequent examination of the fuel filter revealed that it contained several flakes of rubber material. A larger piece of this material had blocked the filter inlet. The source of the material was traced to the partial disintegration of the inner wall of the priming bulb.
