On September 30, 2008, about 1825 mountain daylight time, an amateur built Brennan RV-8 experimental airplane, N272RS, was substantially damaged by an on-ground fire while taxiing for departure at the Idaho Falls Regional Airport (IDA), Idaho Falls, Idaho. The airplane was registered to and operated by the pilot under the provisions of Title 14 Code of Federal Regulations (CFR) Part 91 with no flight plan filed. Visual meteorological conditions prevailed. The commercial pilot and his passenger were not injured. The cross-country personal flight was originating at the time of the accident with a planned destination of Heber, Utah. The pilot reported that while taxiing to the runway for departure, he noticed flames entering the cockpit below his feet. He immediately stopped the airplane and shut the engine down as he and his passenger exited the airplane. The airport's Aircraft Rescue Fire Fighting (ARFF) team responded and extinguished the fire, which was originating from the engine compartment. A few minutes later, the fire reportedly ignited a second time and was immediately extinguished by ARFF personnel. Examination of the airplane by a Federal Aviation Administration (FAA) inspector revealed that the firewall was severely fire damaged and buckled. The engine compartment and forward portion of the fuselage were thermally damaged. Examination of the airplane by a local mechanic revealed that a "B" nut attaching the firewall end of the fuel line from the firewall to the engine driven fuel pump was found "finger tight." When the airplane was placed in a tail high attitude, fuel was observed leaking from this fitting. The mechanic stated that after the ARFF team extinguished the fire, he disconnected the battery. Prior to disconnecting the battery, he noted that the electric fuel boost pump was on. The reason for the loose "B" nut was undetermined.