broken piston ring

[caper_45ca]

Well I found out why I have no compression on one cylinder on my 1995 Johnson 90 hp mod # j90tleor.....toook head off today and looks like a cracked ring.....hard to tell for sure but cylinder wall looks ok....little bit of aluminum scuffing on the side wall but may b able to rid of that with a honing.....hard to tell below piston but am hoping it is ok.....my question is is it a a difficult job to remove that piston.......does the powerhead have to b removed from engine or can start stripping from the carbs in and just remove that one piston....I have a Ken Cooke manual and it describes the process well enough to do it myself...I think...lol....Thank You

 

[Dhadley]

Re: broken piston ring

That should be the crossflow V4. not a big deal. you'll need to get the manual and follow it closely. Yes, you will have to pull the powerhead and strip it down. Even if you could get the piston out, how would you machine it? Or clean it? <br /><br />Once you get it apart youll need the machine shop to measure the cylinders. No matter how "good they look". Once you measure it, that will tell you what needs to be bored.

 

[caper_45ca]

Re: broken piston ring

Finally today the omc dealer called me to see the engine when it was torn down...the other 3 cylinders and pistons looked very clean.....no carbon....the damaged cylinder was beatup very bad as was the cylinder wall...the mechanic said he could find no apparent reason for the ring failure....it was heavily scored on the exhaust side...other 3 cylinders checked out fine..I am having the bad cylinder rebored and fitted with oversize piston and re-ringing other 3 cylinders....dont know if this makes a diff or not but the boat had ngk plugs BZ7HS-10 in it....any suggestions as to what may have caused this problem so as not to repeat would be greatly appreciated

 

[Dhadley]

Re: broken piston ring

dont know if this makes a diff or not but the boat had ngk plugs BZ7HS-10 in it....

Well at least you probably pin pointed the cause of, or at least a contributor to the cause of the bad piston. I'd still check the carbs too. And make sure the top rpm is where it needs to be.

 

[caper_45ca]

Re: broken piston ring

Was hoping you would se my post....thanks for the reply....Are you suggesting that maybe the ngk plugs that were installed contributed to the ring and piston failure..I know the engine according to my manual is supposed to take champion plugs, but the previous owner told me he just installed those plugs and engine was running fine...also mechanic said he checked carbs looking for plugged jet 4 that cylinder but he said carbs were exceptionally clean.....I told him my top rpm with very little weight in boat was 52-5300rpm...35mph...(1985 19' starcraft bowrider) he seems to think that is good???...just a thought my cav plate is level with the bottom of v in my boat with the motor resting against the angle adjusting rod in the hole second from the boat... anymore suggestions?

 

[Dhadley]

Re: broken piston ring

Youve got 2 things, so far, working aginst the longevity of the motor. One is the plugs. They burn slightly hotter than the Champions. Now that alone may not be enough to hurt anything as long as everything else is OK. <br /><br />However, at 5200-5300 your lugging the motor. That also adds combustion temp. Again, all by itself it may be OK for a long time. Couple the two together and the combustion temps are really working against that motor to shorten its life.<br /><br />Then we need to look at any other contributing factors. You said the carbs were very clean and no obstructions. How about the main fuel jet? Was it loose by chance? Most likely it was seated properly but thats the kind of thing we need to look for. How about the inlet needle? Was the tip OK? What about the gasket around the emulsion tube? Were the bowl screws tight? <br /><br />How about the fuel itself? What octane? How old is it? And putting fresh fuel on top of old just means you have all old fuel.<br /><br />The motor sounds a bit low. But that depends on the prop youre using and its condition.

 

[caper_45ca]

Re: broken piston ring

Well ty again for such a quick reply.....as you can see I am a novice when it comes to this...I am having the moptor repaired and do not want to have to do this over again....this motor has power t&t and it gets up on plane very nicely...one thing I should mention is originally this boat came equipped with 1/0...someone manufactured a very professional looking mountout of 1/2 aluminum for an outboard but only drilled 4 holes to mount the motor...they did not allow for any up or down adjustments...anyway I am running a 13 3/4x 15 alum omc blade in new condition....as far as fuel goes all new 87 octane....as for the carb can only go by what mechanic told me that it checked out just fine and supposedly he had it all apart...

 

[Dhadley]

Re: broken piston ring

Ok, good choice on fuel. I think once you get it back together youll be fine. Some adjusting will help. <br /><br />There are only supposed to be 4 holes in the bracket on the boat. The motor has optional holes to allow you to raise the motor (increase the X dimension). Real easy to do. You may not be able to raise it much with an aluminum prop but once its running again we'll see.<br /><br />When its back together and running go ahead and break it in. Once thats done you can establish a baseline. Then we'll move the motor up a hole or two. That way we have our test results to compare to the known baseline.

 

[caper_45ca]

Re: broken piston ring

Well thank you very much for all your help...I will be sure to post as soon as the breakin is over and mayb b4...lol...with all the pertinent information...

 

[Dhadley]

Re: broken piston ring

Sounds like a plan, good luck!!