Airplane was observed to takeoff on runway 13. Shortly after liftoff, witnesses saw the airplane decrease its climb angle or level off at about 200 feet AGL and turn left. The angle of bank steepened until a point about 120 degrees into the turn. The airplane then stalled & entered a steep nose down, left wing leading attitude until ground impact. Winds were reported from 120 degrees at 8 knots. Witnesses stated it appeared that the engine was not developing full power during the takeoff & initial climb. Investigation revealed that the flame tube of the left muffler had separated & partially blocked the outflow from the muffler. This blockage caused back pressure in the exhaust system evidenced by sooting of the exhaust stacks and plugs on the left side. The airplane had received an annual inspection 11 months prior to the accident and there were no subsequent entries. Airplane records did not indicate when the mufflers had last been overhauled or replaced. Pilot's most recent medical certificate was issued on 3/19/79.