rust on top of cylinders 488 merc io

[spareparts]

flip the riser over and look inside the exhaust passage, poke around with a screw driver and see if it pokes thru the inner wall. The manifold is a little tougher, first inspect the gasket surface where the riser meets. Secondly, pull the manifold, block off the hose fittings with some spare hose. level the manifold on a table, fill the water jacket till it almost comes thru at the riser flange, let it sit over night and see if the level drops. If you want to, you can make a block off plate and pressure test the manifold

[cfelton]

Id follow Spares advice! It sounds like a bad riser or leaking gasket. Check Mercruiser Parts Express, thats where I got mine from. Its an original replacement.

[step up here]

look into buying a used motor . there are some good running motors available now at days. you will need more parts then you think once you start on this project. good luck! Break Out Another Thousand spells boat.