I have a 1975 Evinrude 6 hp model 6504 that when put into forward will stay there only at low speeds. When reved it kicks back to neutral. If the shifter is held in forward it runs fine. It also runs well in reverse when reved. I've been told there is an adjustment screw somewhere that can balance the forward and reverse play. I can't find anything around the shifter handle that looks like it could be it. Where will I find this? 