On June 27, 1997, at 1725 central daylight time, a Beech F33A airplane, N234PM, was substantially damaged by a cabin fire during initial climb following departure from Monroe Regional Airport near Monroe, Louisiana. The private pilot, the sole occupant, was not injured. The airplane was operated by a private owner under Title 14 CFR Part 91. Visual meteorological conditions prevailed for the cross country flight that originated from Monroe, Louisiana, approximately 10 minutes before the accident. No flight plan had been filed for the personal flight. According to the pilot, he had picked up his airplane that morning from its annual inspection at Vicksburg Tallulah Regional Airport near Tallulah, Louisiana. The pilot reported that he flew home to Delhi for a short stop and then proceeded on to the Monroe Airport for some avionics repairs. After the work was completed, he departed for Delhi. Approximately 2000 feet agl and 5 miles out, the pilot "detected an odor similar to an electrical burnout." The pilot stated that he detected no onboard problems and decided the smell was coming from outside. The pilot reported that approximately 45 seconds later a flame ignited from behind the fuel selector valve. He "attempted to extinguish the flame with no success." He stated that he "immediately did a 180 degree turn and attempted to get Monroe approach on the radio." The pilot stated that his radios had gone dead so he changed his transponder code to 7700. On short final, the pilot reported that "I opened the passenger door and fortunately this snuffed out the flame." After landing the airplane, maintenance personnel "disconnected the battery, removed the pilot's seat, and removed the cover over the fuel selector valve to be sure there was no smoldering fire." Two loose wires were found next to a hole in the fuel line and "gasoline was running out of the fuselage onto the ramp." The carry through spar was subsequently replaced due to thermal damage. During the course of the investigation, it could not be determined if maintenance personnel who performed the annual inspection or the avionics work had opened the fuel selector valve cover. The pilot reported the accident to NTSB on August 12, 1997. The delay was due to time spent in determining the extent of airplane damage.