'78 johnson 150 hp, low compression on one cylinder

Hello, if anyone could help or clarify. I bought a '78, 150 hp Johnson. Hasn't ran for at least 4 years. I did a compression check and found #1 100psi, #2 90 psi, #3 60si, and the rest right at 100psi. I understand from reading a bit that the next move would be to remove at least the one head(right bank) and check out the #3 cylinder for damage. probably the left head to.

I want to know what exactly I'm looking for, and would there be anything I might find that could be fixed to get the compression back where it should be without a complete rebuild.

I bought this motor for $300 with the understanding that it would probably need a rebuild, I just want to clarify that removing the head or heads is the next move and just what I'm looking for. Thanks.

Re: '78 johnson 150 hp, low compression on one cylinder

The compression on that engine was probably around 110 lbs on each cyl when shipped from the factory. That engine will run fine with compression at 100 lbs. Each cyl should be within 10% of the other cyls, so you have a problem with the one low cyl. You will need to pull the head on that side and have a look. Possible the engine may have been severely overheated and may have damgaged the head gasket seal ring on that one bad cyl. An easy fix. When you have the head off, check the top of that piston for any pits and check the cylinder walls for scuffing (aluminum transfer from the pistons skirts.) If the head gasket is not the problem, you may be headed for an overhaul.

Re: '78 johnson 150 hp, low compression on one cylinder

If it hasn't been rebuilt before that motor should have the high ring pistons and pressure backed rings which are known to roll rings due to crappy gas/fuel additives/cheap oil and cooling problems. If you rebuild it you will find the OEM/aftermarket replacement piston's have been updated.