The pilot reported that after completing a pre-takeoff check of the tailwheel-equipped airplane he taxied onto the runway for takeoff. Upon entering the runway and aligning the airplane to the centerline, the pilot increased the engine power to full for takeoff. Shortly after the airplane began to roll, it started to veer to the left. The pilot applied right rudder, but the airplane continued to veer to the left. The airplane then departed the left side of the runway onto the adjacent grass and struck an airport perimeter fence, resulting in substantial damage to both wings. The pilot reported that there were no preimpact mechanical malfunctions or failures with the airplane’s flight controls that would have precluded normal operation. Additionally, following the accident, a Federal Aviation Administration inspector examined the airplane and found no evidence of any anomalies with the airplane’s aerodynamic flight controls and trim. The inspector also found that a wooden knob that had been attached to the forward seat’s throttle control was lying on the floor of the airplane, but not obstructing the movement of any of the flight controls. The pilot stated that the knob, which did not appear to be an original component to the airplane, had come off just before the airplane had departed the runway surface. He further stated that this precluded him from reducing engine power and stopping the airplane before it struck the airport perimeter fence.