The pilot's delay in recognizing that the airplane was not performing as expected and aborting the takeoff, which resulted in collision with a berm beyond the end of the runway. Contributing to the accident were the pilot's exceedance of the airplane's gross weight and the underperformance of the engine due to governor setting and magneto timing deficiencies. Contributing to the severity of the rear passenger's injuries was his decision to forego use of his shoulder harness and/or the absence of head rests.