Johnson 40 hp Compression

I recently replaced my head gasket because i was only getting 90 psi on the top cylinder and 20 on the bottom. After replacing this gasket it seemed to run much better. But now i check compression to find 60 psi on the top and 30 on the bottom. Is it possible my piston rings are bad? How would i know for sure? If i have to replace the piston rings would i have to bore it any?

Re: Johnson 40 hp Compression

Did you check the head to make sure it wasn't warped before bolting it back up?

What year?

Re: Johnson 40 hp Compression

its a 67 johnson super seahorse 40

Re: Johnson 40 hp Compression

How are you testing compression, which the electric starter? and did you check the head to be straight before bolting it back up.

Re: Johnson 40 hp Compression

you could be lossing compression in three areas: your cylinder head deck surface and your piston.

if you have the option pull you head off and take it to a machine shop where they can check for warp and resurface if need be.

its not the rings that are bad but the actual wear in the cylinder, if you can't get over size pistons, then see if the machine shop will deglaze the cylinder bore. if you deglaze the bore and go together with fresh rings the may help you out it all just depends on how much wear you have in the cylinder. how ever if you are willing to spend the money and have the option the try to get oversize pistons and go over size with the cylinders.

i haven't really tore into outboard two cycle engines but if they are anything like a 2 cycle dirt bike you could have wore reed valves.

well best of luck, hope this can help you out

Re: Johnson 40 hp Compression

i was checking it by way of the pull starter. would it read better with the electric start?

Re: Johnson 40 hp Compression

renken67 said:

i was checking it by way of the pull starter. would it read better with the electric start?

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Yes. Make sure the tester is screwed in snugly and that both spark plugs are out

Re: Johnson 40 hp Compression

i will most definately try that. How long should i turn it over for

Re: Johnson 40 hp Compression

Exactly 3.796 seconds
Really, just a few seconds will do it .

Re: Johnson 40 hp Compression

Well i did it with the electric start and discovered exactly 100psi in both cylinders. Now the starter is bad (had to tap it with a hammer to get it to turn over). I'm gonna take it to a guy i know to have it rebuilt

Re: Johnson 40 hp Compression

renken67 said:

Well i did it with the electric start and discovered exactly 100psi in both cylinders. Now the starter is bad (had to tap it with a hammer to get it to turn over). I'm gonna take it to a guy i know to have it rebuilt

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Nah, just pack a hammer in the fishing kit and hit the water!