2005 Mercury 90 3 cyl scored top cylinder

[homelite]

I have the above S/N 1B173557. It appeared to run fine but had a knock. Pulled it apart and top cylinder is scored. Needs to go .015" over. It's at machine shop now. Other two cylinders perfect. So, from an auto technician (with no inside scoop on 2 strokes), I would like a qualified moderator or marine tech to give me some pointers on a cause. Lean? Oil injection pump? Change jets? It did not overheat and is on a pontoon boat and was never really run hard and has just under 400 hours.

I want to do everything I can to prevent another failure. I know this is common, so I want to know what happened. There has to be a cause. I just don't work on these everyday and don't know the ins and outs of the product.

Hopefully one of you reads this and can give me some reasons and what to look for.


Thanks,

 

[Faztbullet]

Lean or lugged from overpropping...

 

[flyingscott]

If it was knocking did you find all the bearings maybe it ate one. Also make sure the reed valves are all there.

 

[ondarvr]

Not much ino to work with, but....

If just one cylinder is affected then it's normally not the oil injection system, it supplies all cylinders by introducing oil to the fuel line before the carbs, meaning they all get the same amount oil, if one gets less than needed, they all do.

Plugged jet, water in the fuel, carbon build up, lugging (which can be common on a pontoon boat), plus a few other things could do it. Jetting isn't going to solve the problem, the only thing that will is having the shop (or you) determine what failed and fix it. How do you know it didn't overheat?

PS. I have the exact same motor, even the same year.

 

[Texasmark]

My last engine was a '02 90. My guess would be lean running on #1.....low speed clogged as ondarvr stated or screwed in too far, or running without a t stat and the cooling water didn't get completely around the cylinder....top of #1 exposed and over heated. What kind of knock and when did you hear it. Was it a bong or a tick? Cracked ring on #1 a possibility, upper crank bearing due to over heating. OT sensor is 195F and is the tan or tan/lt. blue stripe on the left center of the rear of the block. That part of the block could be under that temp while the top of #1 was way hotter if no water up there as I mentioned.

 

[quicktach]

If you're going over by 0.015 on the top cylinder, I trust you will overbore the others too....for balance.....check with your mechanic

 

[homelite]

I thought the same thing, but the Merc factory manual says no problem boring one as well as the machine shop. .015" not enough to think about as they aren't that well balanced in the first place. It states so in Merc Manual 90-830234R3.

 

[ondarvr]

Pistons re designed to be the same weight so you can mix and match bore sizes.