On August 12, 2000, at 1030 hours Pacific daylight time, a Champion 7GCAA, N9648S, collapsed the main landing gear during a forced landing, following a reported power loss in cruise flight near Newell, California. Neither the private pilot in the front seat, nor the commercial pilot in the rear seat was injured. The airplane, owned and operated by the pilot under 14 CFR Part 91, sustained substantial damage. Visual meteorological conditions prevailed and there was no flight plan for the cross-country personal flight. The flight originated at Red Bluff, California, at 0830, and was destined for Prineville, Oregon. The pilot reported full fuel tanks, 39 gallons, on departure from Red Bluff, which she had confirmed during her preflight inspection. En route the flight encountered up and down drafts that prompted the pilot and her passenger-pilot to divert for landing. Shortly after contacting approach control for flight following, the engine sputtered and quit. The pilot force landed in an open field. She stated that during the landing, the left wheel entered a depression and the left landing gear collapsed. The report of the accident by the local sheriff's office indicated that at 1350, on the day of the accident, there was no fuel spillage or fire hazard present. Personnel who recovered the airplane on August 18, 2000, reported that the gascolator bowl was broken off. Both fuel caps were in place and fit tightly. There was fuel staining aft of the filler necks, chordwise along the top of the wing. After removing the right wing, the recovery crew found about 2 gallons of fuel in the right fuel tank. There was no other fuel found in the airplane's fuel system. As requested by the Safety Board investigator, a mechanic examined the engine on September 19, 2000. The mechanic, who holds an inspection authorization issued by the Federal Aviation Administration, reported the spark plugs looked normal. Cylinder compression was 70/80 or better, except for the number 4 cylinder that had compression of 60/80. The left magneto impulse coupling produced a hot blue spark. All carburetor controls were functional stop-to-stop and the throttle was closed. The mixture was in idle cutoff. Carburetor heat was hot. Intake and exhaust systems were open. The mechanic also noted that the fuel valve was "ON." He discovered the right fuel tank vent was restricted because of a stuck check valve. The Champion service manual indicated that an inoperative vent check valve could result in a fuel differential between tanks.