1981 25 hp Evinrude head gasket

[Boat Junkie]

Hello everyone, I purchased a 25hp evinrude from a guy, he said the top cylinder has 85 psi and the bottom had 120psi compression. I removed the head and the piston head looks good and cylinder walls were smooth, have not been able to look at the rings, the head has carbon build up. Do you think it may be burnt rings? Can I polish the head?

[bktheking]

Almost all used outboards have carbon on the head and piston tops. I'd say clean the head, I use EZ Off oven cleaner to clean carbon, spray it on liberally in a well vented area and leave it for a few hours, it just wipes off, you may need to get stubborn carbon off with a brass wire wheel and something like a dremel tool. I'm not a big fan of short cutting, someone else on here may tell you different but I'd pull that motor apart and check for a stuck ring. You could try the old seafoam trick to try and clean the ring that's stuck. However if you don't want to go to the expense of rebuild, clean up the head the best you can, replace the head gasket and retest compression, may be a gasket leak. Check the head for warpage too.

[kbait]

Since you've got the head off, lay the motor down so the pistons point up and put some seafoam in the low psi cylinder with the piston toward the top. The seafoam should just pool on the top of the piston, and get into the ring grooves too. Clean up the head, ensure that it's flat, and it's mating surface on the block is also flat (flat file works well). New head gasket and check compression again. Basically, just exactly what bktheking said!

[Boat Junkie]

Thanks alot guys i will try both ideas, hopefully i want have to tear this motor down.

What would the pistons and cylinder walls look like if the rings were burnt? This is my first rebuild, i only paid 100 bucks, so if i screw it up i want loose too much.

[bktheking]

Check the cylinder walls for scoring, that's usually a tell tale sign that it ate a ring.

[jasper60103]

Thanks alot guys i will try both ideas, hopefully i want have to tear this motor down.

What would the pistons and cylinder walls look like if the rings were burnt? ...

Post some pics if you can.

[kenmyfam]

Yep, clean it up, check for damage, do a decarb and check compression again.
Good Luck and let us all know.