1973 Evinrude - Variable compression between C1 & C2

[marleywars]

First time boat owner. I have a 1973 50 hp Evinrude Lark on a 1973 Forester trihull. Did a compression test today and the top cylinder topped out at 115psi and the bottom at 142psi. I repeated the procedure multiple times. I have seen recommendations that the cylinders should be with 10% or so of one another. Do I have a concern and is there anything I could do to increase the top cylinder without tearing it apart? Thanks!

Since this previous post (above), I have seen recommendations for using seafoam and changing the head gasket. However, it seems like these may offer limited psi improvements and the underlying and real issue will be with the O-ring on the top cylinder. Obciously I will go with the least expensive route and progress to a more complex resolution.

 

[boobie]

Re: 1973 Evinrude - Variable compression between C1 & C2

Pull the cyl head and check the gasket and the cyl's. That's to much difference.

 

[marleywars]

Re: 1973 Evinrude - Variable compression between C1 & C2

A reply! Great! If I remove the head, I should just change the gasket for good measure right? Doesn't cost much and might as well while I am there? I assume I should look at the head and chamber for gouge/wear and look at the condition of the ring, assuming I can see it. Sorry, just a newbie at this whole outboard thing. Much appreciated.

 

[rrw]

Re: 1973 Evinrude - Variable compression between C1 & C2

Yes, if you remove the headgasket replace it. You might be able to move the piston around enough to tell if the top ring is stuck. Check the top of the piston, the bottom of he head, and the cylinder walls for any kind of pitting or scoring.

 

[Bosunsmate]

Re: 1973 Evinrude - Variable compression between C1 & C2

Put a level across the head in multiple directions to check theres no warping too

 

[marleywars]

Re: 1973 Evinrude - Variable compression between C1 & C2

Thanks for the guidance but one question. Where do I find the six digit part number of the gasket that I see in the Iboats parts for sale section? The evinrude/johnson line appear to have this part or piece starting with the number three.