If you can remove tank, do so and flush with fresh water. That old of a boat, it be a good idea to examine tank now for leaks. Usually, but not always, the crap will float out with water. If you can't remove tank. buy you a cheap electric fuel pump and circulate (pump) gas in tank through filter and back into tank. Acetone will suck up water and may dissolve sludge. I have a feeling that you have a lot of sludge in your tank that must be removed. A portable tank supplying uncontaminated fuel will identify if your existing gas/tank is the source of your head acks.
