1994 90 hp bad compression on 1 cylinder

[curtis45]

Hi folks, need some advise. A friend of mine has a 1994 yamaha 90 hp motor.Last year he ran it out of oil. Ever since then he noticed it not running as good as before and lacking power. Last night we done a compression test, with all spark plugs removed we had 120 on top, 118 on middle, and 60 on bottom. We pulled the head and didn't really see much in the bottom cylinder, a couple real slight grooves but nothing to severe actually we took them right out in a minute or so with a hone. I guess my question is: what do you think might be the compression loss in this cylinder? Broke/ stuck rings? Is there any thing else that could cause this problem? The head gasket look really good didnt see any signs of leakage from one cylinder to the next. Any help would be greatly appreciated.
Thanks
Curtis

 

[Silvertip]

Re: 1994 90 hp bad compression on 1 cylinder

Runing it out of oil obviously is not a good thing and you couldn't have possibly gotten the score marks out at the bottom of the stroke because the area between the rings and piston crown cannot be reached unless the piston is removed. That said, the rings on that piston may have lost their tension as things began to heat up. Reinstall the head and do a leakdown test but there is obviously a problem on that cylinder. As an aside -- siince lubrication failures generally attack all cylinders, this failure may have been partially due to that cylinder running lean due to a partially clogged carb. That in conjunction with the lack of oil may have put that cylinder over the edge (so to speak).

 

[99yam40]

Re: 1994 90 hp bad compression on 1 cylinder

I believe it was Rodbolt that said lack of oil will get a bearing first and the lean fuel/air mix will get a piston first.
My guess is lean fuel mix that ring lands grabbed rings, scored cylinders and pistons.
To completely run out of oil would mean that you did not respond to the oil alarm and safe mode, or by-passed it, or did not keep up with the maintanance and the system did not function as it is suppose to

anyway you look at it the motor would need to be opened and inspected further to see what all is needed. If you see blue metal around bearing areas then it would be a sign of oil loss

 

[robert graham]

Re: 1994 90 hp bad compression on 1 cylinder

It's a pretty good bet that the rings are stuck in that bottom piston. A new piston, rings and a honing might put you back in business, but if you keep running it like it is, it won't get better. Good Luck!

 

[curtis45]

Re: 1994 90 hp bad compression on 1 cylinder

Thank you everyone for your replies. I heard about what I expected to hear. I figured he was looking at a tear down just wanted some other opinions before we tore into it. I guess maybe my next question would be are the compression numbers (120-118) on the other two cylinders seem ok? would it be ok just to replace the rings on the bad cylinder? I know this is not the perfect or correct method but this guy is working with limited funds. Thanks again folks

Curtis

 

[robert graham]

Re: 1994 90 hp bad compression on 1 cylinder

You'll just have to tear it down to tell for sure, but it's probably going to need a new piston and rings and hone out most of the grooves. The other 2 cylinders compression readings sound fine. Another option could be to locate a good used or rebuilt powerhead and install on your motor. Maybe locate and purchase an older, similar/same motor in running condition, keep the old motor for parts or sell it for parts? Good Luck!