1980 Mercury Outboard, 50HP 2-stroke, low compression, help!

Compression as follows....Cylinder 1 = 146 // 2 = 149 // 3 = 145 // 4 = 90 Also, engine will not achieve high RPM's, engine lopes and that tell tale sign of 'white' smoke is coming from the prop exhaust. These are all signs of a blown head gasket, BUT IS NO HEAD.....

Number 4 plug is washed; all other plugs look acceptable, a little on the rich side though. Where is the water and/or compression leak most likely coming from? Took the cylinder block cover off. There are no indications on where the leak might be. Could the leak be a compromised gasket in the exhaust plate-baffle assembly? Or possibly in the exhaust plate in between the power-head and the lower housing?

Any insight would be helpful. Starting to get frustrated with the engine

Thanks

Re: 1980 Mercury Outboard, 50HP 2-stroke, low compression, help!

It might be sucking water in from a bad set of crankshaft bottom seals. This would directly affect #4 and not the others. Of course you'd have to remove the powerhead in order to inspect/replace these seals. With the powerhead off, you can pull the lower main bearing cap without splitting the cases.

It's also possible the leak is coming from the exhaust manifold side, but usually that affects more than one cylinder. You probably won't be able to get the manifold cover off without pulling the powerhead.

Very Very unlikely the leak is coming from powerhead-to-exhaust-plate gasket. I'd bet on the crankshaft seals myself.

If this motor is a Salty Dog, you'll need plenty of heat on the bolts from a Mapp torch (or equivalent) to prevent the bolts from busting off. You really don't want to break stainless bolts in aluminum, it's no fun drilling them out for Heli-Coil inserts!!!!

When you have the exhaust side open, check #4 piston/rings for scoring. If they are badly scored or the rings are frozen into the ring grooves and don't spring back when pushed with the tip of a screwdriver, you're looking at a teardown/rebuild.

HTH & let us know what you find...........ed

Re: 1980 Mercury Outboard, 50HP 2-stroke, low compression, help!

Thank you for your help, it is much appreciated. I will remove the exhaust manifold plates and inspect the piston and rings. Replacing that gasket set is a good idea anyways. I already have the powerhead removed. Underneath the lower end cap are 2 oil seals, an o-ring and a standard gasket. Is that what you are talking about? And yes, it is only the number four piston with low compression. If this is it, u are my savior!

Thank you sir....

Re: 1980 Mercury Outboard, 50HP 2-stroke, low compression, help!

One more question. If the lower end cap assembly is the problem, will it be immediately apparent if that seal is compromised? The reason why I am asking, I would rather service the lower end cap that tearing apart the exhaust plate assembly.

Again, thank you so much....

Jeff

Re: 1980 Mercury Outboard, 50HP 2-stroke, low compression, help!

And another question....is there anything I can hurt by heating up the bolts on the lower seal?

Thank you,

Jeff

Re: 1980 Mercury Outboard, 50HP 2-stroke, low compression, help!

If the seals are bad, it'll be readily apparent. Usually the rubber part of the seal separates from the metal body. Another failure I've seen is when the tensioning spring pops out from around the rubber sealing part.

On the bolts, if they don't readily come loose, you're gonna have to heat them or you'll have a tough time getting them out.

The head shouldn't bother a good set of seals, but if you're going to all the trouble of pulling the cap you may as well replace the bearing cap O-ring and seals while you're at it. They're relatively inexpensive.

Don't forget that you must address the low compression, regardless of the cause you've still got issues inside which may require a rebuild.

You can find parts for your motor here, or try my favorite place, www.boatfix.com

Check out this site for parts diagrams:

http://www.mercruiserparts.com/SelectModelType3.asp?class_id=2

If you should find that the seals and crankshaft lower ball bearing look good, then most likely the water intrusion is coming from the exhaust side.

G'luck..........ed

Re: 1980 Mercury Outboard, 50HP 2-stroke, low compression, help!

Thank you sir for your insight...much appreciated. I am tearing into the engine tomorrow. I will let you know what I find.

Again, thank you...

Jeff