1975 135hp rebuild?

[rockdog57]

I bought a 16 foot crestliner recently with the intention of fixing it up to fish in. It came with a 135 hp Johnson, 1975 vintage. I checked the compression the other day and found two cylinders had 75 lb's and two had 100 lb's. This was with the motor cold. I'm not sure if warming it up will make a difference.
So here's my question. That seems really low for a motor with domed pistons.
If it needs to be rebuilt, is it even worth it? Or would you guys just look for a used motor that is newer and in better shape? It seems pretty gutless on this small of a boat.
I just don't want to put a ton of money into the motor for a 1978 boat.

 

[ezeke]

Re: 1975 135hp rebuild?

There is a good possibility that you have either rusted piston rings that are sticking and/or blown cylinder head gasket(s). You may want to try lubricating the cylinders and pistons and then run the compression tests again.

If you invest in a couple of cylinder head gaskets you can easily inspect the cylinders.

 

[rockdog57]

Re: 1975 135hp rebuild?

There is a good possibility that you have either rusted piston rings that are sticking and/or blown cylinder head gasket(s). You may want to try lubricating the cylinders and pistons and then run the compression tests again.

If you invest in a couple of cylinder head gaskets you can easily inspect the cylinders.

So I should pull each plug and put a little oil in the cylinder then test. Is that going to tell me if the rings are bad?
How much compression should this motor run? I've looked everywhere in the manual and it doesn't give a number. I know obviously it is going to be lower for a used motor.

 

[wilde1j]

Re: 1975 135hp rebuild?

You won't find "a number" for any motor. No one publishes "a number", since there's too much variability. From experience, I can tell 100 psig is OK, 70 is not. I would try decarbing (see Top Secret sticky), see if compression improves, then proceed to pulling heads off. Sometimes, rings get stuck from carbon buildup.

 

[ezeke]

Re: 1975 135hp rebuild?

My 1976 135 ran 130 PSI at sea level consistently. It had the original cylinder heads.