[Sman]

Even if it is fresh water only, eventually the water jackets in the cast iron manifolds do get junked up with rust and can impede flow. I had this problem with one. When I had the water hooked up to the lower unit running it on the trailer, there was a significantly lighter "spitting" of water out of one of my stainless pipes, I also noticed a slight discoloration in that one pipe, (getting hot). I changed out the manifold, cost about 240, got it from a dealer in MA, came with gaskets, did it myself, problem was solved. I was told I should have just done both but did not, oddly enough I have no issues with the older one, (yet). Just one possibility and my boat at the time was 25 years old, lake winni only and had less than 300 hrs, mercruiser.