oil compression test

[2bake]

I have a 1985 70 hp johnson. compression is 125,124&108
now when I add oil to #3 (108) cylinder it comes up about
25 lbs. I know from reading other posts and responces that this is not good. now my ??? What is the most likely problem,rings or cylinder? Is there any fix without
total teardown or disassembly? could I replace rings without removing the power head assy ? Is this old motor worth the trouble?

I'm new to this site and after reading several postings it seems like much knowledge is out there. So thanks in advance for any help I receive.

 

[iwombat]

Re: oil compression test

Start with a decarb (read the FAQ) and see if that improves anything. Your next option is to remove the head and inspect the head gasket.

Lots of things to look at and try before suspecting cylinders and rings.

Adding oil probably removes the head gasket as a culprit, but not necessarily.

 

[2bake]

Re: oil compression test

Thanks for the quick response. Your mention of the head gasket reminds me that also when the motor is warmed up it puts out quite a bit of steam from the 2 little holes
in the aft side near the center of cowling. Is that normal or more indication of head gasket problem?

THanks again,I did try some decard made by OMC.

PS: Hi iwombat, we are almost neighbors, I'm in Kent,wa.

 

[iwombat]

Re: oil compression test

You should be getting some good spitting out those, but probably not steam. Take out the plug on that cylinder. Does it look like it's been steam cleaned?

And welcome to iboats, neighbor.

 

[2bake]

Re: oil compression test

back again. the plug looks the same as the others. we pulled the head and can't see any obvious signs of failure.
Also in looking at the cyls the top (#1) which had compression of 125 has some scratches on one side from the exaust port about 1/2 to the top. #2 looks smooth and #3 (the one with low compression) looks smooth as well.
Could I hone these cyls (in place) and expect the rings to
re-seat? maybe?
We have been doing all we know how to get this thing to run properly. One thing I did yesterday was to pull the plug wire off #3 with eng running (in the driveway) and it ran much smoother. When I did this on #1 and #2 it would die. Looks to me like some blow-by is interferring with crackcase pressure which in turn degrades the performance of #1 and #2.
Also to make a longer story, when we run this eng in the lake it starts fine but when throttling up ( in gear) it faulters and will
die but if the throttle is advanced rapidly it will get past this dead spot and go just fine. then after getting it running fast it can be throttled down to idle and accellerated just fine. However once the eng is turned off for a few minutes, it's back to faultering as before.
We've tried a (temperary) elect. fuel pump to by-pass the vro(oms), new gas lines between the pump and the carbs,rebuild kits, link-n-sink,plugs,power pack,rectifier
and a fuel additive. Hope all this is worthwhile info.

Next input PLEASE .
Many THANKS!!!

 

[iwombat]

Re: oil compression test

Well, we went from slightly low compression to a host of other issues pretty quickly. I say we stay focused on the compression for now.

Remove the side inspection cover on that lower cylinder and see if you can get the rings to move. They might just be all coked up and in serious need of a decarb.

You can't hone anything in-place, don't even go there.

 

[2bake]

Re: oil compression test

Well now! It's starting to look more like a big anchor but
I got that plate and the exaust manifold off. The rings are all springy when I push on them. Is there supposed to be a specifc amount of movement? There is a collection of carbon at the exaust manifold exit. Is this from the decarb
I did or just from old age?
Any more ideas on what to do next?
I sure do appreciate the quick response and involvement.
I've never seen the inside of a 2 cycle before so I need all the help I can get.
Thanks again again!

 

[2bake]

Re: oil compression test

Hello iwombat,
Hope you haven't given up on me yet.
Like I said all the rings move and are springy but after looking some more I find that the bottom(nearest the crank) ring does not move as freely as the larger top one.
This is true for all 3 pistons. Also,when pushing on the bottom rings it seems that area near the upper access moves more then the bottom area. Hope to hear from you again soon. In the mean time I'll be cleanig things up and get ready to put it back together. If I don't hear otherwise I'll probably put it together and do more decarbing.
Thanks again for your help !