It was thought to cool the exhaust on motors pushing a heavy load. On a light boat the exhaust is so efficient on those models that the exhaust cavity runs at negative pressure. This pulls water from the normal outlet and makes you think it’s not pumping. I sold quite a few after market overheat warning light kits to concerned customers. They bolted to the cylinder head and included a push button to check the batteries (two "D" cells) The sending unit could be tested in a pan of heated water. It was supposed the turn on over 190 degrees F. Thermostat equiped two cycle outboards run around 150 to160 degrees F. . . 🙂
