The pilot reported that during the takeoff roll, as the airplane accelerated to 40 mph, the engine sputtered and he reduced power with sufficient runway remaining to stop; however, the sputter ceased and he then added power to continue the takeoff attempt. The engine sputtered a second time and the pilot reduced engine power again. At that point, he was concerned about the length of runway remaining, and elected to steer off the left side of the runway, rather than risk going off the end of the runway and down a ditch. As the airplane departed the left side of the runway, it struck a runway sign and came to rest upright. During the accident sequence, the airplane sustained substantial damage to the aft fuselage and empennage. The pilot subsequently defueled and moved the airplane before it was examined by a Federal Aviation Administration inspector. As such, the inspector was unable to verify the fuel quantity or if any fuel contamination was present. His subsequent examination of the airframe and engine did not reveal any preimpact mechanical failures. Additionally, the pilot reported that there were no other preimpact mechanical malfunctions and that he should have aborted the takeoff earlier.