Where's the compression

[dakk1]

1985 Merc 150<br />Been sittin up 5 years, rebuilt water pump, carbs, fuel pump, new gas etc. Cranked it today on the muffs and it seemed to run fine even idled pretty smooth after a while. Then I get this constant beep beep beep and I shut it down. Put my hand on the engine and its hot but wont burn ya. So I figure nothing major here. So I decides to check the compression. I cold tested it when I bought it and all cylinders about 105 or so. Now when I check it top left cylinder is about 15 psi. Very bad I say to myself. Others are 115 to 120. So I pull the head to see what the bad news is, and I don't see a thing. Cylinders are smooth as a babys behind. No broken rings, nothing. Turn the flywheel by hand observing cylinders, everything appears normal. Where's my compression? Oh yeah, gasket if fine.<br />Thank you one and all for any advice...Dave

 

[Hooty]

Re: Where's the compression

If everything else looks o-k, it could be the rings are stuck. 15psi sounds kinda low for just stuck rings though. Do a decarbon job on it and see if the compression comes up.<br /><br />c/6<br /><br />Hooty

 

[dakk1]

Re: Where's the compression

I will do the decarb. I think Hooty might be right about the stuck rings. I went back out and turned it by hand and it seems as if the other 2 cylinders on that side are "cleaning the oil film off the sides of the cylinder as they travel. On the problem cylinder a portion, maybe a third of the cylinder still has oily film on it after the piston has gone by.<br />If I run this motor with poor compression on one cylinder will I damage something else.

 

[yz250]

Re: Where's the compression

Sounds like your rings are melted to the piston usually caused by a lean cylinder overheat. Sounds like you'll need a new piston and rings

 

[Hooty]

Re: Where's the compression

I think I'd do a decarb before I did anymore running. You'll have to run it to decarb but I would limit it to that. Doesn't take long.<br /><br />c/6<br /><br />Hooty

 

[dakk1]

Re: Where's the compression

Glory Be! Decarbed with Mercs Powertune and the cylinder is now testing at 112 psi. The rest are at 115 to 118 so I'm pretty happy. Also used the original gasket. I'll put on a new gasket now that I won't need a rebuild and might improve some more. Thanks to Hooty and YZ250 for the help....Dave

 

[Hooty]

Re: Where's the compression

Sierra Hotel!<br />Glad to hear it.<br /><br />c/6<br /><br />Hooty