That model operates at a normal temperature of 150 to 160 degrees.The water circulates through the system until the thermostats begins to open at approximately 140 degrees. Until that happens, only the water bypassing the thermostat via the notch in the thermostat housing will be seen at the outlet in the back of the exhaust housing. When making idle adjustments to the carburetor, the access door is open and the motor is running on ambient temperature. It may take five minutes after that access door closes before the temperature stabolizes and the new adjustment is realized.This is true when the motor is shut off for any length of time.The choke is far from perfect and poor idle from lean operation is normal until the temperature stabolizes. The underwater exhaust outlet on that motor is very efficient and it will run with negative pressure at speed on a fast hull. This means 90% of the cooling water exits the motor under water and very little is seen at the exhaust relief outlet at the rear of the exhaust housing. This lead to the installation of the overheat warning light, since visible water was no longer a reliable warning of cooling problems. This page is from the 1964 Lark service manual. . . 😉
