Force 90hp low compression help

[Daphne444]

I have a 1990 90HP Force that was giving me some trouble with stalling when shifting into gear last year. Anyway, a recent compression test showed 110/80/110. I suspected a scored cylinder in #2 so I pulled the cylinder head to take a look.

The good news is that all the cylinders don't seem to be scored. I cannot feel any binding spot with the fingernail test anywhere on the side walls. However, the #2 piston itself doesn't seem to be in very good shape. The piston top is very pitted/dinged up and over at about the 11o'oclock position, near the exhaust ports, the edge of the piston looks like it has been pried up (see picture).

Pistons #1 and #3 have a much better top surface, however they also have the "pried up" small section at the edge in the 11o'clock position.

Any thoughts on what this means? What causes the "pried up" edge on the pistons? #1 and #3 still had pretty good compression even with this feature.

Thanks

The three separate pistons are shown in the photos. It is pretty easy to identify #2.

 

[pnwboat]

Re: Force 90hp low compression help

The only thing that I can think of that would cause the "pried up" portion of the piston from looking at your pictures is a broken ring. Could also be a broken reed valve but seeing how the problem area is at the same position on all 3 pistons, I would suspect broken rings. That number 2 piston looks like it was over-heated. Maybe due to a lean fuel mixture.

 

[JB]

Re: Force 90hp low compression help

Well, it appears that some sort of debris has pounded the top of piston #2, and that it has tried to catch on an exhaust port.

Could be something pumped through the crankcase, like pieces of a reed, or it might be bits of piston ring.

80psi seems like a lot of compression for a cylinder with even one broken ring, though. Maybe the debris was carbon chipped off an exhaust port or released from a ring groove.

Pix 2,3,and 4 confuse me. Are they 3 different cylinders or have you just offered three more pix of #2? They all seem to have the same damage at the same spot but the crown damage doesn't show up on 2 of them.

 

[Daphne444]

Re: Force 90hp low compression help

Guys, thanks for the comments. I see I was not clear about the pictures, sorry. Pictures #2,3 and 4 are of all different pistons going from top of the engine to the bottom. The damage on each at the 11o'clock position was something that surprised me since they all looked so similar.

I have been doing more reading and found a couple of posts that show some similar damage on pistons, but not on all of them in the same engine like I have here. What could cause that?

I suppose that I have to replace all three, even though I was still getting decent compression on two of them?

thanks