Need advice on troubleshooting low compression

[Wee Hooker]

Hi all, been a while since I've been around but could seriously use some advice on my current project. I have a 90's vintage Mariner 4hp one lunger that has low compression. (about 35#). (Got it froma guy that said it "just died". I've tried adding oil to the cyl to see if compression would go up (due to worn rings) but it doesn't. I've also pulled the head and replaced the gasket. Still 35#. While the head was off I inspected the cyl wall and head and saw no sign of a crack . I'm running out of thoughts on what to do next.
Is there a good method to look /test for micro cracks? Could it be the reed valve broken/open? It's a nice little motor so I hate to give up!
Thanks
Dave

 

[racerone]

The reed valve has nothing to do with low compression in the cylinder !-----Even with a reed valve missing there will be good compression in the cylinder.----I would test with another gauge first.

 

[Sea Rider]

If you can turn flywheel by hand with min or no restriction at all while spark plug is fitted that OB is shot. It's unrecovarable no matter what you do. When too low on compression OB will be difficult to start and eventually will die soon afterwords. Go for next size piston & ring kit, cylinder will need to be bored to a larger diam at a machine shop. Besides the kit will need to order a gasket kit if everything else is found in excellent shape.

Happy Boating

 

[Wee Hooker]

Thanks again, I actually just found and posted to the thread I had on this motor in 2016 in the Tohatsu section. ( It was sitting in the basement all this time.) I can't turn the flywheel by hand but I do believe it needs rings now. Not sure if I'm up to that task as It would be new ground. May get a quote to get it done or just parts the motor out to support my 2, 5 hp Tohatsu/Merc sweeties.

 

[Sea Rider]

If replacing just standard rings for new ones, no way OB will recover it's full lost compression, just slight more PSI and that's it.

Happy Boating