Just a general question.

I'm not a mechanic but do my own work on my outboard because of necessity. A few friends know that I do an admirable job of keeping my 20 year old clunker running and occationaly ask me to look at theirs. Twice lately and once on my own outboard before I rebuilt it 5 years ago I have come across this problem and wondered what is the most likely cause of it. The other day a friend who was having trouble keeping his 99 Johnson 115 running asked me to look at it. The first thing I did was do a compression check. Then I had to give him the bad news, 115 lbs in 3 cylendars and 85 in the other. I pulled the heads off to show him the damage and of course found the top cylendar scored. What I found interesting is that the cylendar head looked the same as the last two engines I found with a bad cylendar, all beat up. What causes this? Or should I ask what gets in there to beat them up like that.

Re: Just a general question.

It sounds like the rings broke from coking. <br /><br />Are you sure its a 1999? Way more common in a crossflow than a 60 degree looper.

Re: Just a general question.

It is a crossflow I think, had a bubble exhaust cover anyway. I'm not sure the year, thought he said 99. Anyway it doesn't matter I'm not working on his just found the problem for him and since I have seen this 3 times just wanted to know what causes it. Thanks.

Re: Just a general question.

The rings get stuck in the groove from a sticky carbon-like stuff. As the rings coke up they stick in the groove and the piston looses support. The rings start to stick on the back side oppsite the end gap first. As the piston looses more and more support it will start to rock and eventually shear off one side of the ring end gap.<br /><br />This will happen, generally, on the starboard side first. If you look at one of these pistons with a broken ring, holding it up so you are looking at the ring locating pin, you will see that most likely the ring end to the right of the pin broke first.<br /><br />Several things contribute to this coking. We generally find that those motors were set up to turn in the low to mid 5000 rpm range. <br /><br />Motors set up to turn 5800 to 6000 rarely if ever have this happen.