Diagnosing low compression issue ('95 35hp johnson)

Hi everyone,

The motor hasn't ran for sometime and after cleaning it up it started fine the first time and has not started since(low compression now).

The top cylinder 60psi, and 90psi in the bottom(checking it cold since it wont start now). I took off the bypass cover and looked at the rings on the top piston(its a pretty limited view). From what i saw they looked okay, I gave them a poke with a screwdriver and they were not seized. I can't get a great view of the cylinder walls either but again from the limited view they did not look scratched up.

Is there anywhere else the low compression could be caused from, or does it boil down to rings and cylinder wall?

I would pull off the head bolts to take a better look but a couple of the bolts are so rusted I know at least a couple of them will snap so I rather look everything else over. Thanks!

Could be a blown head gasket. You'll have to pull the head.

You'll want to look at the tops of the pistons too, good luck with those head bolts.

Swarlos said:

You'll want to look at the tops of the pistons too, good luck with those head bolts.

Click to expand...

What should I look for at the top of the pistons? Just carbon build up right?

And thanks I am 100% certain a couple of them will snap, they're in bad shape. I am debating if I should just trade it in at a local marina or take it to a shop to verify if its the ring or Head gasket.

Head gaskets don't normally just blow. They usually blow from previous overheating or attack by shade tree "mechanics". Or salt water eating away the metal.

I don't think a shop will be able to verify what's causing low compression without attacking those very same head bolts. You'll want to see if there are any chips or dings or anything on the tops of the pistons. I don't think you'll get much money trading in a motor with low compression. Why are the head bolts so rusted anyway?

I am not sure why they're so rusted. I picked it up for next to nothing and the guy didnt know anything about it, not sure how he treated it.Before I picked it up I checked the compression it was fine and after I started it and ran for a few it wouldn't start again.

It's one of those MARS outboards that was made for the military back in the day so who knows what it has been through.