The non-certificated pilot was conducting visual patterns the airport. While in the pattern, the airplane's engine began to sputter and lost partial power. The pilot examined the clear fuel lines and saw air bubbles in the fuel lines. The pilot attempted to prime the engine to clear the air bubbles, but was not successful. The pilot steered the airplane towards the runway in an attempt to land quickly. The airplane could not maintain level flight, so the pilot began a descending turn for the runway. Once the pilot realized that due to strong winds he would not make the landing surface, he elected to perform a forced landing to an open field. While maneuvering to land a gust of wind increased the angle of bank and the pilot lost control of the airplane. The airplane impacted terrain nose low with approximately 45 degrees of bank. Substantial damage was sustained to the outboard portion of the left wing. The airplane was flown by the pilot and the previous owner prior to the accident without any problems. An examination of the fuel system and fuel lines did not reveal the source of air in the fuel lines.