On November 10, 1993, about 1852 eastern standard time, a Cessna 421, N1600B, listed by the FAA as being sold to Spec Industry and Commerce Ltda of Brazil, operating as a 14 CFR Part 91 personal flight, experienced a fire in the vicinity of the left wing during engine start. Visual meteorological conditions prevailed at the time and an IFR flight plan was filed. The airplane was substantially damaged and the airline transport-rated pilot and one passenger were not injured. The flight was originating at the time of the accident. After the fuel tanks were filled, the lineman noticed a fuel leak under the left wing and advised the pilot of this. The pilot told him that the fuel leak was because the fuel tanks were overfilled. The airplane was then pushed forward and when the pilot was starting the left engine, the engine backfired and an explosion occurred, followed by a fire near the engine nacelle area.