I have finally completed the rebuild of my son's Doodle Bug. Upgraded the 2.5(?) HP for a 6.5 Predator. Everything looks to be lined up, and when we rode it in the back yard on grass, it worked fine. As soon as my son took it out on the paved road, the chain started popping off. I put spacers between the rear axle and sprocket to square the front sprocket with the rear and keep the motor centered in the frame, and the chain looks to be square. The teeth don't appear to be overly worn on the sprocket. I am at a loss. Any ideas will be helpful!!!