The airplane contacted soft terrain and trees during an off airport forced landing following a loss of engine power. The pilot reported he was making "slow banking" turns to the left during the flight in order to maneuver around controlled airspace. He reported that during the flight the engine began running rough at which time he noticed the fuel flow dropped from 90 pounds per hour (pph) to 70 pph with a manifold pressure of 24 inches. The pilot reported he turned on the fuel boost pump and switched to the right fuel tank. He stated that the engine power increased, but the fuel flow was still fluctuating between 110 and 80 pph. The pilot stated he then heard the engine stop at which time he switched the fuel selector back to the left tank. The pilot reported the engine regained power, but the fuel flow continued to fluctuate from 85 to below 70 pph. The pilot reported the airplane could not maintain level flight and he began looking for an off airport landing site. He located a large, lit parking lot and descended toward it. During the descent, the pilot realized he was not going to be able to make it to the parking lot, so he located a nearby clearing in which to land. The pilot reported he confirmed the landing gear were extended and he added 10 degrees of flaps prior to landing. He also turned the fuel selector to Off and shut off the master switch prior to touchdown. The nose gear collapsed after contacting the soft terrain and the wingtip contacted trees during the landing roll. Post accident inspection of the airplane by inspectors from the Federal Aviation Administration, St. Louis, Missouri, Flight Standards District Office, failed to reveal any mechanical failure/malfunction that would have resulted in the loss of engine power. The inspectors reported they drained less than a pint of fuel from the right fuel tank and approximately seven gallons of fuel from the left fuel tank. The inspectors also reported there was no fuel present in the fuel lines from the fuel pump to the fuel flow valve. The pilot reported he departed with 15 gallons of fuel on board the airplane and that the accident flight lasted approximately 10 minutes.