Ole’s first row boat motor used a pressure actuated intake valve. He called it a check valve carburetor. Later he added a float bowl so you didn’t have to adjust for varying amounts of fuel in the tank. Others used the piston skirt as a valve. These "third port" outboards were prone the back fire through the carburetor when run at slow speeds with a lean mixture. Third port motors in particular spit back into the intake passage at an idle. Flame arrestors put out the fire by reducing the temperature of the charge using aluminum or copper. Johnson countered this backfiring by adding aluminum flame arrestors to the bypass passages in the cylinders. They all added air horns to direct and contain the fuel and flame away from the boat. Some had flame arrestors on the front of the carburetors. The rotary valve increased efficiency by opening and closing sooner, but could also backfire if run lean at slow speeds. At high speed the motor benefits from a longer open time because of the ramming effect of the velocity of the incoming charge. At idle speed this is minimal to nonexistant. The light and effective reed valve overcomes these deficiencies by being faster acting than the check valve and at the same time (unlike the third port or rotary valve) automatically adjusting to the various engine speeds. These improperly adjusted motors (Lean adjustment backfiring) are the leading cause of broken reed valves. (In my opinion) Second is running over speed.
