[Delta-T]

UPDATE:

The new engine was actually smaller than the old one (despite the ~2x HP) and so the height differential between the exhaust manifold and the muffler has decreased by a few inches, and due to the new 3" very stiff hose we need for a 90 deg. elbow immediately below the exhaust elbow -- all of which means that the angle of the hose running to the muffler is much shallower than with the old engine. My estimate is that it's a 8-10" drop from the exhaust manifold to where the hose enters the muffler, and that the hose is 10-15 deg off horizontal.

If I am reading this correctly, you have no riser connected to the exhaust manifold? I made this same mistake when I replaced my exhaust system. Last year I added the riser and have not had an issue since.

This is before with no riser:

And this is the modification:

 

[Delta-T]

Bill, how old is your boat? Old enough to have a re-power done to it. My 1969 Bristol engine is at or below the water line. The old exhaust was not a wet exhaust and needed no riser. The pipe was a one piece, double walled brass pipe. Exhaust in the inside pipe and water in the outer jacket. See attached picture. The hard mounted engine and one piece exhaust pipe must had been installed as the boat was built around it. When I rebuilt the engine, the exhaust pipe was not serviceable. I had to remove it in three pieces. The reason I rebuilt the engine was because I got a hole in the exhaust pipe, water into the exhaust and back siphon into the engine. I replaced the system with a wet and did not do the riser, the riser was needed as well as a siphon break elbow just before the water goes into the exhaust. You can see that item in the last pictures.

So, one of two things for you to ponder...what was your old exhaust system and do you have the siphon break elbow?