97 350 vortec engine knock only under load?

I have a 1986 286 searay 350 mercruiser with alpha outdrive. Engine was replaced a few years ago with a 1997 350 vortec engine. 2 weeks ago i had a pretty bad overheat. Melted the exhaust couplers. Replaced the water pump, exhaust couplers, shift cable. Risers and manifolds looked ok. Started engine up on the hose at home and found engine wanted to lock up when idling. Drained the exhaust manifolds and removed the risers and found water in the manifolds. So ordered and installed new manifolds and risers. Removed the water from the cylinders and drained and refilled the engine oil. Changed oil and filter 3 times till i got nice clean oil. This weekend while out after running the engine for about 2.5 hours on and off plane several times. When the wife went to get up on plane the engine just shut off and wouldn't restart could smell gas thou. Got towed back to our slip and found i had no spark. Coiled failed went and got a new coil. Took about 20 minutes of trying till she started. Ran real rough for about 5 minutes or so. just thought cylinders were loaded up with fuel. The next day we head back out. The first thing i noticed is there is a very slight vibration coming from the engine just thought it was still loaded up with fuel yet. So she goes to get the boat on plane at around 1800 rpm's the engine starts to knock like a rod bearing. Back off the throttle put the boat in neutral and shes tacs up to 4K and no engine knock. Put back in gear and as soon as it hits 1800 rpm's the engine starts to knock again. If i have a bad rod bearing shouldn't the engine also knock when revving it up? But she is quite all the way up to 4K just when there is a load on the engine she starts to knock. I've attached a link to the you tube video and u can hear the knock towards the end of the video. just don't want to pull the engine just to find there is nothing wrong with it. I did check the oil and it's nice and clean yet so the block or heads didn't get cracked from the over heat.

https://youtu.be/UddTQygYwxc

Sounds like a wrist pin, that'd knock under a load. Eventually it'll start doing all the time.


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I had some time today to mess with the boat. Found the timing is all over the place. Boat was hard starting today. Once i got it started and warmed up i checked timing at an idle. It was at 8 deg then 32 deg. It will not stay at one spot. Plus the tach was bouncing around between 500 rpm's and 1500 rpms and u could hear the change in the way the engine was running. At 2k rpm's the timing mark still isn't steady it's all over the place. I'm gonna try replacing the pickup sensor in the dist. It already has new coil,plugs,wires,cap and rotor

Don't know if this applies to your engine but on some electronic distributors you have to install a jumper to put the unit into base timing mode. What you describe sounds like the dist. isn't in base mode. A new pickup won't hurt anything.

Try the pick up first. It's cheap and easy to swap out.
Do you still have a thunderbolt IV ignition? If so, and the pick up doesn't do the trick, it could be the module.
That module is what brings the timing all in.