Low Compression on bottom Cylinder (#4) 1989 Mercury 40

[Blueraven1959]

I have been told that I have low compression on my #4 cylinder of a 40hp 4 cylinder 2 stroke. I check all the compressions and they are as follows.
#1 #2 and #3 all have about 123psi. The #4 has 75. I was told the engine was not worth fixing and to get a new one. I had a friend come over and we put some oil in #4 cylinder and the compression came back to 123psi. So my question is what is causing the low compression. It has been suggested to put Marvelous Mystery Oil in the #4 and let is sit for 2 days which I have done. With out tearing apart the engine, is there a way to get this compression resolved. I am mechanically inclined but never have taken an engine apart.

 

[Faztbullet]

Usually this is because the lower crank seals have failed and water has enterend cylinder,,,it scuffs the piston so only cure is to rebuild.

 

[Blueraven1959]

Is it a big job to replace the lower seals? What is the proceedure to rebuild it. Is it a BIG job and costly?

 

[Faztbullet]

Seal aint going to fix it,,you will need to tear it down and inspect cylinders and piston,hone or bore oversize and reassemble. On a 1-10 scale about a 8