The airplane was on takeoff roll and witnesses reported it was near the end of the 3060 foot runway, when it lifted off. According to the witnesses, the airplane climbed slowly and struck trees about 2000 feet past the end of the runway. A witness who arrived at the accident site shortly after the crash encountered the pilot who asked him to turn the ignition switch off. The witness noticed the switch was set at the left magneto position. The pilot had no recollection of the sequence of events associated with the accident. The investigation included an operational test of the engine. The engine would not exceed 2400 RPM on both magnetos, and no more than 2100 RPM on the one magneto. A check of the engine to magneto timing revealed the magnetos were timed about 10 degrees outside the specified timing parameters. The engine was retimed to specification and it was operated to 2500 RPM on both magnetos, 2400 RPM on one magneto.