'85 Johnson 140 v4 possible piston ring issue?

[indyfiero]

Hi guys,

I picked up a project over the weekend with the idea the engine was toast and needed piston rings due to low compression on the bottom starboard cyl. After talking to the seller when I was picking it up he informed me that the engine was "acting funny" and felt "low on power" when he had it out about 4 months ago. He took it to a mechanic who did a comp test and said he had 3 cylinders at 125 psi and one at 115. They then pulled the head off the starboard side and the mechanic told him the piston rings were shot and needed replacing and would do a full rebuild for $2200.

When I got the boat home I pulled the cowl off, removed the head(which he had just loosely put back on with the old gasket) and checked for damage. There is no evidence that it blew a ring from the top side. Cylinder walls look fine on both pistons, no chunks missing from the head and the ports seem unscathed as well. There is a TON of carbon in there though.

Is it possible this guy is wrong, is there something else I should look for? Is it worth throwing a new gasket on it and running some seafoam through it to break up the carbon?

I'm new to boat engines so please forgive me if I missed something obvious here.

 

[racerone]

Model # helps.

 

[indyfiero]

Ah sorry. J140TLCOS or C0S, can't tell if its a 0 or a O.

 

[Faztbullet]

That's a smqll bore looper and it has the bridged exhaust. Bad t-stats(to cold) builds carbon as cylinder temps never reach peak to burn it off ands the bridge port has a tendency to bow in and scuff exhaust side of piston, cylinders will look fine but piston/rings will be scuffed. Also the pistons have 2 small holes drilled in their side that helps cool the bridge. The 1985 has one of the worst carb sets so don't expect a smooth idle

 

[indyfiero]

Thanks Faztbullet. Is it worth trying the decarb then to see if that blows out some of the carbon and restores a bit of the lost power and compression? I'm hesitant to tear the thing apart based off one cylinder reading 10psi lower compression than the others. If it is scuffing the rings, any idea how long I have before it eats the ring completely? Is there any way to get a look at the exhaust side of the piston without taking the whole thing apart?

 

[Faztbullet]

Yes you can try and decarb it. I am not a fan of the "internet"overdose method as seen to many customers motors fail shortly after they did it. I prefer to use a OEM(Yamaha,Merc,BRP) spray and trim the motor up,remove plugs and rotate crank and soak 2 cylinders at a time overnight, then crank and empty can thru carbs.If the bridge has scuffed the piston/rings the scuffed area will only continue to get worse.This is a foam cast block so no cover to remove and inspect.