How do you flush a 1974 40hp Johnson outboard?

The earlier model 40 hp Johnson and Evinrude outboards use a special flush adapter. It fits over the water pickup screen in the side of the exhaust horn, located under the backside of the antiventilation plate, just above the propeller. I doubt that this flusher is still available on the market, so probably your best bet would be to use a large garbage barrel. I have, and on occasion still do, use this method when serviceing the older J/E outboards.