I believe you should be able to remove the cowl and see a rubber tube going down to the bottom corner of the engine pan. It will be in the rear of the pan close to the corner on the shift lever side of the pan.
Good luck,
JBJ
A 1975 does not have a tell tale. Water will exit from holes just above the anticavitation plate when the thermostat opens. Also a 3/8 inch hole on the back of the motor just below the hood latch. Spray will come out that hole when the thermostat opens if on the water or in a barrel. No water will come out there on muffs. It needs exhaust back pressure before water will come out the exhaust relief hole.
Make sure the thermostat is working. On muff start the motor and put your finger on the top of the block behind the flywheel. Should be able to hold finger there about 4 seconds with out getting burned. When thermostat opens water should come out the holes on top of the cavation plate. It should be warm to the touch. Thermostat is 140 degrees so if have a heat gun can also use that.
My 1975 15 Hp has had 3 or 4 bad thermostats and each one has stuck open and the motor ran cold. My motor spends about 50 percent of it time in Salt water.
