low compression on one cylinder

[Thumpbass]

Well I've been dealing with this low idle miss for a little while and finally borrowed a compression tester from a friend...did what I should've done in the first place

Found that on my '79 Johnson 100 I have 7 psi on my starbioard top cylinder, 95 psi on the bottom and 120 and 125 on the port side cylinders. I'm no mechanic but I'm pretty sure this is probably a pretty bad scenario

My motor seems to run fine in every othrt situation except for idle so I have 2 questions

1. Does this sound like a rebuild to fix it and
2. What risks am I running if I just leave things the way they are if it's good enough to get me on the fish?

Thanks

 

[R.Johnson]

Re: low compression on one cylinder

It could be just a head gasket, pull the head, and see. Don't run it that way.

 

[Thumpbass]

Re: low compression on one cylinder

It could be just a head gasket, pull the head, and see. Don't run it that way.

what would I need to look for? Are there some other things I should inspect while it's off? I have never taken a head off so as much info as I can get would be much appriciated

Thanks

 

[Silvertip]

Re: low compression on one cylinder

Cylinder wall scoring and piston damage. If those look ok, replace the head gasket and recheck compression. If its up considerably, consider doing a decarb. Rings may be stuck due to carbon build up. Do a search here. Lots of info on how to do it.

 

[Thumpbass]

Re: low compression on one cylinder

Cylinder wall scoring and piston damage. If those look ok, replace the head gasket and recheck compression. If its up considerably, consider doing a decarb. Rings may be stuck due to carbon build up. Do a search here. Lots of info on how to do it.

If the cylinder walls and pistons look good would that suggest that I don't have damaged or broken rings or can you have damaged rings and no cylinder/piston damage?

 

[RPJS]

Re: low compression on one cylinder

You would be one lucky dude if you had busted rings and no piston/bore damage.
7psi is a big leak, if you had damaged the rings/holed a piston I think you would have been aware of the problem, it certainly woudn't of happened quietly, my guess would be head gasket.
When you pull the head try to keep the gasket in one piece any blow in it should be obvious when you look at it.

 

[Thumpbass]

Re: low compression on one cylinder

I've never pulled a head so bare with me...Do I need to pull the head cover and check that gasket too or should I just pull the head and leave the head cover (and gasket) alone?

 

[Silvertip]

Re: low compression on one cylinder

The head seals the combustion chamber. The cover seals the water jacket. Both need to come off. You can't see the piston and cylinder until the head is off. Even if the cylinder and piston look ok, the rings can be junk although if they are broken, cylinder scoring would be evident. That's why I suggested installing a new head gasket and checking compression again. If it comes up you solved the problem. If not, all you spent was a little time and a few bucks for the gasket. Last advice -- get a service manual.

 

[Thumpbass]

Re: low compression on one cylinder

Thanks, I'm about to call my parts guy and see if he has the gaskets...I have a manual so I'm good to go there...thanks again guys! I'll report back

 

[Silvertip]

Re: low compression on one cylinder

While you are at it, remove the plugs and compare the plug on the low cylinder with the others. If it is very much lighter in color than the others, you may have a lean carb condition that contributed to the failure. Check the overtemp sensor and warning horn to be sure they are working.

 

[Theoutdoorsman]

Re: low compression on one cylinder

Thumpbass................ I was affraid of that man. I really feel for you. But don't get discouraged. It's hard not to, I know. But keep your head up. Judging by your report on the compression test, it looks to me like you still have decent compression on the other three cylinders. That #3 cylinder is a little questionable though. A good honing and re-ring might just bring that back up where it needs to be. Just to let you know, mine did the same thing. My #1 was toasted also. But you know what, it really isn't that hard to repair the damage. It just takes some time for us newbies. I went all out when I rebuilt my 140, and it did cost me quite a bit of money, but you can re-use some, if not most, of what you already have and get through this pretty reasonable I think. I just opted to go all out because I planned to keep mine for as long as it will hold together. I would be willing to bet you have a considerable amount of carbon build up, as these crossflow motors are bad for that. I wish I would have know this beforehand. I'd have done a decarb and ran seafoam religiously!! That #3 piston probably has stuck rings from this carbon build up, hence the lower compression on that cylinder. I know you already have the OEM Service Manual, so no need in suggesting that again........ d ........... Read it, and then read it some more to familiarize yourself with what your looking at. Popping that head off is no problem at all. It only takes a few minutes to do. The head cover can come off later. I don't know how much help I can be to you, but I'll help where I can as I'm sure others will likely jump in and rescue you as well. Your in good hands. If/when you disassemble, be absolutely certain you keep EVERYTHING in little baggies and marked so as to reassemble the pieces of the puzzle EXACTLY as you removed them. This can't be stressed enough. If you need boring, and don't have a reliable shop in your area, Dhadley suggested Mar-Fab to me. I can give you their address and telephone number so you can talk directly to them. Very very nice people to deal with! You can also buy whatever parts you might need from them. Go ahead and pop that head off and let's see what we're looking at. I'll cross my fingers for you, maybe it IS just a head gasket. Get back with us on this......................... ALAN

 

[emdsapmgr]

Re: low compression on one cylinder

Two other things you can do to inspect for damage. All it will cost you is some time and a couple of gaskets. Remove the side cover from the bad cylinder and see what the rings and piston look like. Ditto for the exhaust covers. If you are going to do a rebuild, these have to come off anyway. If not, you saved yourself having to remove the block from the midsection. Also, when you get the head off, look for pits on the top of the piston and head surface-sure sign of a ring that let go. I would probably leave the head cover on the head, just remove the head-with the cover intact.

 

[Thumpbass]

Re: low compression on one cylinder

no such luck on the gasket...the top starboard piston is toast. It has the pitting mentioned above as does the head. I hate to say it but I don't have the skills, tools or really the time to rebuild this thing....not to mention I have had a little issue with a small amount of water making it's way into the lower unit and a leaky trim cylinder as well so I'm kinda skidish about dumping any more money into this motor whether it's a rebuild kit or a rebuilt power head. I may look around for a used power head for a few hundred bucks or may be a little more on a complete motor that will give me a few years of service until I upgrade to a bigger boat (when the kids are all old enough to go out and fish)....thanks for all the help guys, may be I'll be back sooner than later....

 

[Theoutdoorsman]

Re: low compression on one cylinder

I'm sorry to hear about your misfortune Thumpbass. I know it's like getting a big kick in the teeth. Before you get rid of the outboard PM me.