Low compression, 90 hp Mercury

[scotth1968]

I have a 90 horse Mercury 2 stroke outboard and I am getting compression of 45-50 lbs of compression on all 3 cylinders and I would like to know if the gasket is blown or maybe rings. My friend stated he was in the water and the boat just shut off and he had to have it toad into shore. Please advise if anyone as any input on what might be the problem- Thanks !

 

[Bondo]

Welcome Aboard Scotth,... I moved yer post from this thread to yer own thread here in the outboard forum,....

With compression like that, that motor needs a full on rebuild, she's Wooped,.....

 

[ondarvr]

Not every gauge reads the same, so I'm always suspect when I see low readings that are fairly even over multiple cylinders. Other than that, since they don't have head gaskets, there's probably a good deal more damage inside.

 

[Texasmark]

Why did you check compression? Do you know how to check it? Do you know if your gauge is accurate? No head gasket as I recall. Check the top (rear actually) of the block. If you have one gasket and one cover....that the spark plugs are sticking out of, you have no head gasket.....the head is an integral part of the block casting....if you have 2 gaskets and 2 covers, the center one is the head and you possibly could have a blown head gasket, but the likelihood of all 3 cylinders being affected is highly unlikely.

One thing could be gummed up rings.....that engine uses pressure backed piston rings that have to be free to move about in the piston grove. Go to the archives or just google search the words "De Carb", or outboard decarbonizing or words to the effect. Step by step instructions on how to do it and what to expect.

"Don't fix it if it ain't broke".

 

[racerone]

Remove exhaust cover , about
20 bolts and have a look at pistons and rings.------Is it still oil injected or is the system bypassed ?----Has water pump been maintained ??