Compression????

[Pst76]

Doing routine prep for new season I decided to do a compression test more as curiosity than any operational issues. I have a 1991 Johnson Ocean Runner 150hp. All cyl are reading 100psi with the exception of #3 cyl which is running at 50psi. Test was done with motor warmed up. Engine sounds great no skip would never have even questioned a problem. Now that I have checked.... I sprayed some engine tune up into carb for that cyl last night per instructions on can. Will be taking it out for a run tonight. Hoping that maybe somthing is just stuck, ring, reed? Any thoughts<br /><br />Thanks in advance

 

[what66]

Re: Compression????

Try spraying a little 2cycle oil in cyclder<br />and see if it increases if it does it's your rings.<br />have you tried de carbing?

 

[Pst76]

Re: Compression????

I'll try that.

 

[Dhadley]

Re: Compression????

I sure wouldnt run it under a load (in the water). Id run it on the flushette for a few minutes and the recheck compression.

 

[Pst76]

Re: Compression????

DHadley, Thanks for the heads up. How quickly does the decarb work?<br /> Would save me a 30 min ride to the boat launch if compression is still low after that. Also should oil be sprayed into cyl or just a small squirt. I know it wouldnt take much. I should be able to determine problem without leaving the driveway, correct?

 

[Dhadley]

Re: Compression????

If you used Engine Tune from BRP, I wouldnt let it set in there very long. Overnight is way plenty. Fire it up and let it run. Youll smell it. When it quits smelling then recheck the compression. Usually running it about 10 minutes or so will do the trick.

 

[Pst76]

Re: Compression????

Well here it is, Did the decarb which worked fine, not much crud came out but seemed to be some. Tested compression and still same readings all cyl 100psi except # 3 at 50psi<br />Added about a tablespoon of 2 stoke oilthrough plug hole and tested. Same reading 50psi.<br />Cannot see anywhere external where head gasket may be leaking so Im thinking either hole in piston or internal gasket leak. Next step is to pull the head. Any other thoughts first?<br /><br />Thanks

 

[Pst76]

Re: Compression????

At this point assuming that one piston is bad. Would I be better off to tear the whole motor down and rebuild or fix only the problem one. Also what should I look for that may have created the problem. I had two issues last season one with motor, voltage reg burnt up and gas filter o-ring broke causing low fuel/restriction.<br />Any inside will be appreciated

 

[Dhadley]

Re: Compression????

That motor does not have head gaskets. A fuel restriction is not a good thing. <br /><br />What plugs were you running?<br /><br />Go ahead and pull the head off.

 

[Pst76]

Re: Compression????

DHadley, I pulled the head off last night and found that the cyl had a spot where carbon had stuck to the side wall. About an area the dia of a thumbtack head. Also there is some scoring on the opposit side of the cyl wall. Piston top is clean as new and rings seem fine. If I try to wiggle the top of the piston it always springs back to center. Looks like I will be tearing the motor down. Interesting thing is that the plug is burning clean and inside the head as well is perfect. Not sure what happened but will need to fix. I did check compression on that cyl one more time before I pulled head. I installed compression tester on the bad cyl and fired motor up for just enough to start for a second and then shut down. Bad cyl pressure jumped to 90 psi.<br />If all goes well I will start tear down this week end. Lot of work for just one cyl so plan to do rings on complete motor.<br />Oh, Plugs are standard recommended for that motor,I had confirmed that on the site when I got the motor, I forgot number off top of my head but they are Champions <br /><br />Thanks