if your engine compaetment is wide open and make sure you have no gas fumes i have started the engines in the boat with the manifolds off to see if the exhaust guides are leaking..it will make like a steam effect when it gets hot...just extend the hose that comes off the thermostat housing and dump it over board..

the problem with that is it wont make any pressure and very little heat on the garden hose.

I saw you said you pressure tested the manifold, but did you also pressure test the tailpipe/riser? Cold water dripping on a hot exhaust valve will take it out. Ive seen the stainless marine tails have pinholes at the welds near the bend where they turn to go towards the transom.

I think today i will swap sides with the manifolds and risers to seee if the leak will follow the riser and manifold. That may confirm if the head head a problem.

Just swap the risers if you can.

Have you tried running the water for a while, and NOT starting the engine? Then remove riser, and look for water traces dribbing down. If the engine isn't running, water should not get near the riser joint if the head is leaking. If you can, try to rig the garden hose directly to a freshwater flush if you have one, or directly to the 1 1/4 line feeding the engine. So it can build a little more pressure than the muff's will allow. You may have to remove the water stat(so water flows thru block) and just let the hose run for a while and see what happens.

call dave at imco get new set of power flows no more water

Well I think it was water reversion due to to big of camshaft and riser gaskets leaking. I cleaned up the gaskets and put some silicone on them and lengthened my pipes on the risers. Ran it for a while took the riser off and all was good. Where can I get good riser gaskets for the stainless marine exhaust? They are like a flat gasket that gets tightened into place with the v-band clamps.

Im confused. Can you post a pic of your exhaust manifolds? Stainless marine manifolds should be a dry joint between the riser and manifold. Meaning that water doesn't flow thru the joint, just exhaust. The water is bypassed from manifold to riser. If water was leaking from the riser/manifold joint, somethings wrong.

What camshaft are you running in this engine??

You are correct but with the short riser it must have givin the ablity to pull some water back into the manifold. Also I was told that if the riser wasn't sealed it would create a vacuum and cause reversion. So I lenghtened my pipes closer to the tips and sealed up the riser with some silicone on both sides of the gaskets. Ran it for a while pulled the riser and no reversion. Some told me that the overlap on cylinder #5 could have be alittle greater than the others on that side. It seems to be cured but I want to get some good riser gaskets and check it again so if its good I can just put on the good gaskets and be done.

Corey, my heads were tired on my 468's almost dropped a valve. I chose to go to Dart Iron Eagle heads. The Chevy castings are basically unchanged from 30 years ago and don't fix the issues they have. I would strongly suggest looking at new heads.

Wannabe