I was warming up my motor yesterday on the hose (HP500 carb, sort of) running it at about 1500 rpms, brought it down to idle for a few seconds ready to shut it off, and it start knocking badly and locks up

After i cleaned my shorts and stopped crying, I decided to pull the drive. Drives fine. Took out all the plugs, fine. Oil is clean. So while taking off the manifolds (Merc/Gil), I notice the port side manifold runners have water in the bottom of the runners, all 4 of them. I assume the manifold is cracked.

My question is, how are these pressure tested? I plugged up the outlets and clamped a garden hose to it. I have about 80 lbs here at the house, but I can't make it leak, they look fine.... Where the hell did that much water come from?

Did you actually pulls the intake and heads and look inside. You sure that's not just condensation etc? The crack would be inside to do that and not that common. Maybe you smoked a bearing.

Is this a new engine? Meaning, did you just put it in the boat? Is it possible that it is reverting water, or has this engine been in the boat for some time?
The riser may be leaking. You really can't pressure test them, but you can run water through it and still check it for leaks. You also need to pressure test the engine basically the same way you tested the manifold.
Eddie

Haven't pulled the intake or heads yet. Not condensation, it was a lot of water in the runners. Boy I hope it's not a bearing... I just can't imagine a bearing going while at idle.

I built this motor last winter. Put 40 hours on it over the summer, and it ran flawlessly. I'll run some water through the risers and see if I can find anything.

I'd bet the riser/elbow went bad. What did the manifold/riser gasket look like?

If not those, then the only other answer is reversion. You could have hit some harmonic holding it at 1500 that sucked water down, but the exhaust pressure kept it from entering the exhaust valve. Once you dropped back to idle, there would have been much less exhaust gas velocity coming out of the cylinder, with the exh valve staying open longer, allowing it to flood.

normally when running on the garden hose,the volume of water is low,but reversion is possible,if you had no reversion problems for 40 hrs of previous running,id bet it is a bad manifold or riser,did you pull the plugs and check for water in the cylinders?

If it has 40 hrs without any reversion, then it about impossible that it just developed it for no reason.
Did you maybe have the tongue of the trailer really low, which would have changed the tailpipe angle? That could cause water to run up the pipe if you were normally really close to reverting. It wouldn't take much to send it over the edge if it was close. Just a thought.
Eddie

P.S. Very sorry to hear this happened. I've done stuff like this, and it is sickening. Did it to a diesel that had a sketchy home-built exhaust w/o enough riser loop.

Eddie, I was thinking the same thing, but deleted it before sending because I figured most normal tailpipes are 13-14 degrees down perpendicular with the transom.

That said, when I had wet exhausts, I always made a habit of cranking up the tongue as high as it would go before running on the hose just out of principle. That's because I F$*(#ed that diesel I mentioned above due to running it without enough angle. (There's an article floating around on TheDieselPage.com about that Duramax project I did back in '04).

No. It was sitting in the same place as always. Actually about level. Been doing it there for years and never had a problem like this