I guess all my analysis did not make it. So far have changed the spark plugs, oil, both head gaskets(which did not appear bad). Drilled two relief 1/2 " holes in exhaust. Engine had carbon build up in heads. Checked compression 85 and 95 on each piston. The motor is 26 years old with 1600 hours. It does not blow out the relief carb vent until about 4 min of run time. This seemed to have happened suddenly and not a slow process. Hate to give up that old Cub Cadet 580 she has been a real work horse for me.