Cause of low compression

miket

Seaman
Joined
Aug 11, 2003
Messages
59
Johnson 115hp 1998, trouble starting and idling.<br /> running hot. Replaced water pump. <br />Top right cylnder 95 other three at 110<br /><br /><br />Removed head to replace gasket to find damage to top right piston and head. Appears a chunk of metal dislodged and banged around dinging the head abit, and gougint the piston. Appears to be a chunk of the edge of the piston. I can see top ring which appears in tack, but the piston around this edge looks deeply damaged.<br /><br />What should I do?
 

sikpnter

Petty Officer 2nd Class
Joined
Jun 21, 2003
Messages
137
Re: Cause of low compression

Not much of a choice here buddy sorry. How bad is the cylinder wall damaged?
 

R.Johnson

Rear Admiral
Joined
Sep 24, 2003
Messages
4,446
Re: Cause of low compression

You are probably looking at a broken ring here. The only fix would be a rebuild. Depending on how much damage to the cylinder wall. If you do a rebuild, you will want to get to the bottom of the problem that caused the damage. Lugging the engine at to low a RPM would be one cause. You will want to look over the entire system on a rebuild, carb's, sync, and link, ign, or let's just say the works. We can help you along with yor questions. If you have to, you can go .040 over on a rebore if you use Wiseco pistons. Let us know as you go along.
 

miket

Seaman
Joined
Aug 11, 2003
Messages
59
Re: Cause of low compression

^The cylnder wall look good, Can I just replace the one piston?<br /><br />RJ what does lugging engine at low rpm mean?<br /><br />Should i buy remanufactured powerhead?<br /><br />Mike
 

R.Johnson

Rear Admiral
Joined
Sep 24, 2003
Messages
4,446
Re: Cause of low compression

If you are running a prop with to much pitch, it won't let the engine come up into it's power band, or let's say, it's proper RPM range. It would be like pulling a heavy load with your car in to high a gear. This is what lugging means.
 

Solittle

Fleet Admiral
Joined
Apr 28, 2002
Messages
7,518
Re: Cause of low compression

Yes Mike you can replace just the one piston. After you have it apart take it to a marine machine shop. They will bore the one hole. Oversized pistons are available and this is a common repair.
 
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