How low is too low compression

[shadowrider]

I have a 96' 15 hp merc 2 stroke motor, ( came with a boat purchase). It has low compression on both cyls. 50 and 55 , I can not get it to fire of course. I am getting spark and fuel, but without compression I think it will not run. My thinking is I have stuck rings, if i could get it to run I would seafoam it. I pulled the inspection covers and see carbon buildup on cover and a scorched looking piston. No scoring evident. Any thoughts ? Thanks

 

[Chris1956]

Re: How low is too low compression

Tilt it up and lock it. Now remove spark plugs and fill a cylinder with Seafoam, wait and pull it over a few days later and see if compression improves. Repeat for second cylinder.

 

[shadowrider]

Re: How low is too low compression

I did that but only let sit a few hours. My understanding is that seafoam works best when heated by a running engine. If I was to do it overnite, Should I bring piston up near tdc so it will hold seafoam and do the same on other cyl a few days later ? Thanks Cris1956

 

[shadowrider]

Re: How low is too low compression

I filled one cylinder with seafoam and let sit 3 days, no change in compression. I used regular Seafoam, is it the same as seafoam deep creep ? If not would deep creep do anything different? Thanks

 

[Maxz695]

Re: How low is too low compression

Try PB Blaster it may help losen things up, Work it in well turning the flywheel back and forth with the plugs out then put the plugs in to compess it ito the ring section of the piston let it set at least a day. take plugs out and crank the engine over til it disperses then retest for better compression. May have to repeat this several times

 

[cr2k]

Re: How low is too low compression

To answer you original question 55 is WAY too low compression. As evidenced by the fact that both are low and close I would almost guess worn out or maybe overheated at some point. But do try what is suggested it just may help. Anything below about 95-100 is considered low compression depending on the engine of course.

 

[shadowrider]

Re: How low is too low compression

Actually, the compression is at 60 now, so not a lot of improvement. I just picked up some pblaster, will give it a whirl.