I just purchased a 91 carver 370 aftcabin about 2 weeks ago. I had a survey done and he said it was in great shape. He noted a few things in need of repair but nothing major. I havent taken her out other than a sea trial and manuvering to my new slip in the marina. I'm still familiarizing myself with the boat and the systems. This is my first inboard and sealed cooling system but i understand it pretty well or so i thought. Anyway, i started the starboard motor and after warming up it immediately overheated. Checked the obvious first, i put my hand on the risers and noticed that the starboard side riser was at an acceptable temp but the port side riser on the engine was pipeing hot and i could hear the air in the lines bubbling. Air is not good and tells me there's a leak. After looking around and checking fluid capacities i noticed a faint rust drip at the bottom of that hot riser. After looking closer i could see it was cracked and had rusted itself closed to some extent. No water coming out but I'll bet air was getting in. I pulled the riser today to prep for getting a new one and i was surprised to see that it had a gasket that was purposely on there to close off all the 4 outer ports in the exhaust manifold??? I could clearly see antifreeze in one set of the outer ports and i assume the other 2 are for the raw water to exit throught the exhaust because there is a 90 degree hose fitting leading from the thermostat housing that connect to those ports. Why then were they all closed by the gasket? I could see the gasket was made to do that but why...is my question? I need to get a new riser but i need some info b4 i go out to get it. Any info will help...thanks in advance.... Steve