The pilot/owner, who used the aircraft regularly from the strip, reported that the engine started without difficulty and the magneto checks were normal. Power was applied for takeoff on the grass strip adjacent to Runway 21. During the takeoff the pilot became aware that the aircraft was not accelerating or climbing. After briefly becoming airborne the aircraft settled into a wheat crop just beyond the Runway 16/34 intersection and pitched over inverted. The pilot speculated that the engine may have been producing insufficient power. He considered a mechanical failure to be unlikely in view of the easy starting and satisfactory magneto checks, but considered the possibility of carburettor icing following five minutes ground running at idle power, or that he may not have applied full power due to a stiff throttle linkage.