85hp Low compression

[Kevin58145]

Hey guys at the end of last season I checked my compression and if I remember correctly the numbers were 125-90-125, which bothers me. So today I got a new toy for the tool box(bore scope) and wanted to check it out so I pulled the spark plugs and by hand rotated each piston to BDC and put the camera in, The top bore looks good noting that cought my attention other than the carbon build up, The middle bore looks good as well just has the same carbon as the top, The bottom bore looks pretty good there were a couple of questionable marks but the scope makes a business card look like its 1/8in thick and the questionable marks were way less .
Also the bottom bore is a .020 over piston the top two cylinders apper to be stock with no markings is this normal for only one cylinder to be oversized?

As for the low compression on 2nd cyl could this mean that the rings need to be replaced?

I've heard of decarbing the motor but have never done it, Does it work? and could it be the reason for 35psi differance between cylinders?

Im comfortable doing my own engine work although this is my first outboard 2stroke (well 1st 2stroke ever).
I've built several small block chevys from the ground up but have never touched a 2 stroke till this boat

by the way its a 1988 85hp force

since I got it 3 years ago I've changed the spark plugs a couple times rebuilt carbs last year, changed the gear lube every season and the impeller last year and a fuel pump kit,

What other maintainance should I be doing I wanna get it running top notch before going out this year.

thanks in advance I hope I get lots of replys.

 

[pnwboat]

Re: 85hp Low compression

The rings have to be able to "float" in the ring grooves in order to seal properly. If too much carbon builds up in the ring groves themselves, the ring will seize and won't seal properly. You can't see this looking at the tops of the pistons. If you want to look at the rings, you can see a small portion of the side of the piston/rings if you take the by-pass covers off. You can see the side of the piston and rings through the intake ports in the side of the cylinder. I would try a de-carb first to see what affect it has on the compression. De-carb (hopefully) breaks up the carbon build up in the ring grooves.

It's not uncommon to see one cylinder oversize while the rest are standard.

 

[Kevin58145]

Re: 85hp Low compression

Thanks for the reply I'll do the decarb when it gets a little warmer here in michigan maybe even this weekend weather man says its gonna be in the 40's this weekend,

does over size pistons have any power gains in these motors?

also how long do the reeds usually last?

 

[pnwboat]

Re: 85hp Low compression

You typically won't see much gain in horse power by putting in oversize pistons. At least nothing that is really noticeable.

The reeds on the older motors seem to last forever. I have a set in my 1988 125HP that are still perfectly good.