1999 3 cyl mercury - engine or drive unit knock

[dmbuz33]

Newbie alert!!
I bought a pontoon last year that was parked for a few years. After a thorough power washing (4x), fuel tank cleaning, water pump and seasonal maintenance I used it a couple times last year and I've put maybe 3 hours on it this year. The noise you hear in the video has been there since the first time I started it and while I'm not incredibly familiar with outboards, this noise seemed to be in the drive unit so I figured I'd check it out later. It's a small lake and I always had a tow if needed.
Well, I started looking at it a little closer and searching around on the internet today, I really haven't found a noise exactly like this one but I do have a theory that I got from reading an old post here.
I would love some input from the guys that know these things well.
Some inspections I've done today.
Compression test- all cylinders 115 psi
I've checked for any additional clearance in the crank train the best I can through the spark plug hole. Everything seems okay. The engine runs well, idle quality sucks a little, probably some carb cleaning needed. This noise is directly related to engine speed- in and out of gear and is audible all the time until the noise of riding in a boat drowns it out. I'm really hoping someone has heard this noise before and nails the diagnosis.
Thanks for the feedback.

 

[CharlieB]

I don't like that noise at all.

Remove all spark plugs, turn the motor over by hand until #1 cyl is at TDC Top dead center. Now turn the flywheel by hand while feeling the piston position with a small screwdriver or piece of wire. You want to find out just how far the flywheel turns before the piston begins to move. Now do it again only turning the flywheel the OTHER direction. You are wanting to see just how much piston DWELL there is at TDC.

OK, now do this again on cyl #2

And again on #3

If there is a bad rod bearing then it will show as a longer dwell before the play is gone and making this knocking noise.

If I am right. If I am wrong you are just out of some time and a bit of effort.

 

[dmbuz33]

Thanks Charlie,
This is something I did before I posted the video but I didn't completely erase that as a possibility so I dug into checking the rods and pistons a little deeper today. I hot-glued a degree wheel to the flywheel and used a dial indicator to measure the piston movement. I'm pretty sure that I've eliminated connecting rods as the noise source. (I guess I could have a broken piston skirt) Measuring the same piston movement in each cylinder results in exactly the same crankshaft rotation. I used .010 each side of TDC and has exactly 6 degrees each side of TDC.
I read a pretty old post about the possibility of cylinder liners moving on 40-60hp engines that were over-propped or over heated. With my limited time driving this boat and using an online prop calculator, it does appear that my engine is over-propped and lugging ~ 4600 RPM. I think it was Faztbullet that posted about the slipping cylinder liners. Have you or anyone else ever heard the noise that makes? It does seem like a slipping cylinder liner would lower the compression due to the increased volume of the chamber. All of my cyl were 115 psi.
I have some fuel system parts coming that will keep me busy a couple days but I'm still interested in anyone's suggestions on this noise.

Thanks,
Dave

 

[Faztbullet]

Just got thru building one that was doing same thing...knocking. All cylinders has was above 116psi except #3 which was 102psi. This is why compression test are only good for seeing if cylinder can support combustion and is not a good indicator of engine condition. Leakdown test on that cylinder was over 35%!!!! Bored cylinder,honed and replaced all pistons,brgs,gaskets and run like a champ.

 

[dmbuz33]

I still suspected a piston issue just because I think the noise has a double knock sound to it. Thanks for the input. Estimated cost for rebuild of a 3 cylinder 40 hp Merc?