The pilot had just departed the runway when he reported to the flight service station that he had engine problems. The aircraft was observed by witnesses to be about 300 ft on a descending downwind. The pilot stated that he was circling for runway one. Surviving passengers described the engines as running rough and uneven. The investigation revealed that the aircraft's weight was more than the pilot had calculated and the C.G. was 3.4 inches further aft than was calculated. However, the weight and C.G. were within limits. Examination of the engine disclosed that the right engine had an extensive cylinder head crack, a partially disconnected intake pipe, and was capable of producing 55% of rated power. The left engine had seven severely worn cam lobes. The rudder trim was deflected full left at impact. The evidence indicated that the pilot had retarded the throttle for the left engine and was using only the right engine to sustain flight. Exam of company checklist usage revealed several different improper versions.