Check feeder drum arms for binding where bolts go thru to adjusters. Have had a few that when the spring loaded adjusters are set correctly, the chain is tight on one side and almost dragging the floor on the other side.When you remove the drum assembly and set it on the floor, one arm touches the floor and the other arm is off the floor 1/4 to 1 inch! Remove the arms and lightly grind the hex shaft to let both arms touch the floor. Next check the arms to be parallel . Mine were closer together at the bolt end than the hex end. Bend in a press till parallel. Surprising how much tighter it lets the chain run . IMHO, the arms fit too tight on the hex anyway, not allowing the movement and tensioning during wear that the 88 series allow.