During initial climb at 300 feet above ground level, smoke began to enter the cockpit area, emanating from the windscreen defroster vent. The pilot elected to return to the airport for landing on runway 22. As he flared for landing, a gust of wind lifted the right wing, and the left wing then contacted the runway surface. The airplane ground looped, which resulted in substantial damage to the left wing rear spar and left elevator. Recorded winds at the airport, about the time of the accident, were from 240 degrees at 11 knots, gusting to 14 knots. Subsequent inspection revealed that two large holes in the muffler were the source of smoke. Prior to departure, the pilot reported the engine backfired during his magneto check.