Hi. 1974 evinrude 70 hp outboard. was working great. I switched from one tank of fresh gas to one with half fresh and half year old gas. ten minutes later the boat started quitting when I had it in fwd or rev. Ran fine in idle. When I switched back to the first tank it still quit, but maybe I need to let it run for an extended perios of time to flush out all the bad gas? Not sure if the old gas was treated or not. I am assuming this is bad gas. Anything ELSE it could be?