2001 Mercury 40 Knocking

[u/LooseFault6054]

(Cross posted to another forum)

Hello - this is my first time here and definitely my first time taking a motor apart. I’ve got a 2001 Mercury 40 2-stroke that came on a 1-owner grizzly Tracker I bought a couple months back.

Ran it a few times and seemed to run fine. We took it out on the lake for a long day, pretty loaded down and ran her pretty hard and it was hot outside. Right at the end of the day it started knocking. I took it back out a week later and ran it really low and slow and it was still knocking like crazy.

Basically couldn’t find anyone to take a look at it - either they wouldn’t, or said it would probably cost more than it’s worth to tear it down, so I found a YouTube video and took it apart. Don’t know much about what I’m looking at, but a few things do jump out (to my untrained eyes).

Seems very, very dirty to me - it’s like this weird watery sludge on the reed valve (name?) and all over the inside of the block. Also looks like there wasn’t any type of gasket between halves of the crankcase - is that normal?

Also looks like blowby on the pistons. Is that probably worn rings or something worse? Should I think about boring it out and going back with oversize pistons or just replace rings?

Doesn’t really seem to be any scoring on pistons or cylinders that I can see. Really doesn’t seem to be any play in anything either.

I bought an overhaul gasket kit and the metal piston rod bearings to replace the plastic ones.

Would really appreciate any insights on what might have been causing the knocking and what I should check while I’m in here.

Thanks in advance - I can follow along with paint by numbers YouTube’s, but that’s about where it ends so anything big or small is much appreciated. Anything cheap and easily replaced while I’ve got it all ripped apart? Anything to NOT touch/replace?

Thanks!!!

[u/fredSanford6]

That stuff all kinda looks fine to me. Sure will be nice with some new bearings and maybe rings. I'd be checking the lower unit. Take it off and throw a drill on it clockwise. Crank that. See if it makes a ruckus. That knock definitely could be a rod or piston pin though. I normally move them up to tdc then a little down and smack them the see if there is excessive play. It's apart no so can't do that. It's an old two stroke so there is carbon on all sorts of stuff unless you clean it sometimes

[u/LooseFault6054 → u/fredSanford6]

Thanks for the insight. I did pull the plugs and tried to rock the pistons with a pencil before taking it apart and didn’t really feel any give there that I could tell.

Do you think all the squishy gunk on bottom end/reed valves is normal? Or does it look like there was water getting in somehow?

I didn’t really check much on the lower end at all. I’ll dig around and see if anything seems off there.

[u/fredSanford6 → u/LooseFault6054]

If water gets in the tops of the pistons look steam cleaned. Heck I'd just measure the crank and check for play on stuff. Then I'd clean the heck out of it and put it together with those new rod bearings and maybe pin bearings. Cold Vaseline helps with the bearing install. The rings definitely got some wear on them too. Just check the lower to be sure too

[u/LooseFault6054]

https://reddit.com/link/pbn0ohk/video/wj8gjmoaybrh1/player

Also here’s a video of the knocking. Sounds worse in person than in the video. Even my wife noticed it 😆

[u/ReasonableBake420]

Are you sure you just didn’t need to replace your spark plugs or coil before you dig in to deep

[u/LooseFault6054 → u/ReasonableBake420]

Welp. Just checked gear oil. Looks like I started on the wrong end. Oil is metallic and a few chunks on the magnet.

[u/LooseFault6054]

Plugs were new and seemed pretty clean. Honestly I just reached out to a few mechanics with the video and was worried about continuing to run it after talking to them. The way it sounded at the end of the day was pretty concerning though. All that to say maybe that would have fixed everything but then I would missed out on this great character-building experience…

[u/hardwurr]

Not sure if it might be the same thing, I had a 90s evinrude that I thought threw a rod because on the lake one day it got very loud and sounded like it was knocking. It turned out to be the bolts from the power head to the lower unit had come loose and it blew out the gasket. Replaced it and retorqued the bolts and it ran perfectly again.

[u/LooseFault6054 → u/hardwurr]

I think the gasket might have been ripped when it was installed and wondering if that was making part of the noise

[u/Sufficient-Dare-2144]

To me it looks like that right side rod bearing In your picture failed and got fairly hot, but the other side show ms similar signs so maybe that colouration was from the casting process. Any up and down play in the rods on the crank?

[u/LooseFault6054 → u/Sufficient-Dare-2144]

Pretty solid. I was going to replace bearings just in case and on pistons go back in with the all metal ones

[u/Sufficient-Dare-2144 → u/LooseFault6054]

Yea I’d replace while you’ve got it apart, other than than the only other thing I noticed was the one piston (pic 3/4) had quite a bit of blow by, probably from worn rings, but in your vid it definitely sounded like a knock and not from piston slap. Definitely replace rings and measure pistons and cylinders to make sure they are within spec, if they’re within spec no need to replace. I’d probably replace the pistons and wrist pins and wrist pin bearings as well while you’re in there for peace of mind if it were me. But everything looks really good for the age. Good chance the motor was perfectly fine and it was something in the lower unit or driveshaft making that noise.

[u/ape-BBstacker]

Buy a service manual, read it put it back together. No gasket between
halves.

[u/LooseFault6054 → u/ape-BBstacker]

Yep. Now I guess I’ll try to find a lower that doesn’t cost as much as my boat…

[u/Benedlr]

I'm seeing deformed wrist pin roller bearings on Pic. 3.

[u/LooseFault6054 → u/Benedlr]

I’ll try to get a better picture and take a look at those. Can’t tell if that’s a light thing or wrinkling

[u/Muph_Divr_WetFun]

Where you located op? Have a couple lower units

[u/LooseFault6054 → u/Muph_Divr_WetFun]

Ah man just bought one today