TOPIC: moisture in number 2 cylinder.

I purchased a 1979 1/2 Merc 115 outboard for my project boat. She ran really good. never took her over 5500 rpm and always pissing strong, but I didn't have a temp gauge working at the time, normally playing around 4 to 5 thousand rpms.

I got a 22 pitch SS prop and got 50mph out of her at 5k rpms on Saturday. I tried to raise the motor up another half an inch and noticed it was not pissing as strong so I lowered it back down and quit trying to go faster at that time. Sunday I put the 15 pitch prop on it, lowered the motor way down, and went to the lake to play with some family and friends pulling a small tube. Temp sending unit installed and never came above warm.

I did have a hard time starting her at the dock and a fellow boater gave me a shot of ether and she fired up and seemed to run good. Took a warm up lap nice and easy around the lake, beached the boat and met up with my friends. we discussed getting the small tube and letting the kids play on a tube for a while so I let the graduate drive the boat back to the dock. her first time driving any boat. she did great. No issues, started right up, ran pretty good, but idle in gear was a little bit rough. I was thinking it maybe needed a little carb tuning, no biggy.

Got the tubes and started to play. The motor started to run a little rougher at idle, seemed to be smoking a bit more and running a bit rough like maybe a little rich. stalled a few times in gear at idle, but ran and pulled the tube with no issues after I put it in neutral and revved her up a little bit, seemed to clean it out and ran better.

I just wanted to give some back story to my issue so to my point now. Today I pulled the boat out of the garage and figured I would see if I can get her to idle better adjusting the carbs a little bit. While I was filling up the 55gal barrel with water, I figured I would pull, check, and clean the plugs. number 2 plug had a milky sludge on it. I believe I understand these motors dont have heads or head gaskets. So there is no head gasket failure to look for.

Compression numbers.
1-115
2-120
3,4,5-125
6-120

What is the bad news here?
Did I hurt this motor some time?
Where do I look for damage?

After a quick tune up of the carbs and got her idling much smoother. I pulled number 2 plug back out. I dont like the looks of this.

not a merc guy but i'd say you have a leaking gasket. check out marineengine.com.

That gasket (#22) is a water jacket gasket, not a head gasket. Water can't get into the engine there. I'd remove the exhaust cover gaskets & look there.

The 1979.5 115 is an inline-6,...Not related to the posted image by Jim.
Ideally, replacement of gasket 14 "might" resolve the issue, but I strongly encourage replacing 14, 17, & 19, plus insuring the "water Jacket Cover" #13, is not warped, or corroded,etc.
The "Exhaust Manifold Baffle Plate" #18, also has to be carefully inspected for corrosion pits/holes, too.
This is not an easy task, and short-cuts will commonly result in the water intrusion rearing its ugly head again.
If you see cracks between the exhaust ports, forget spending money on the beast.
doc

Hoping for some more advise from the Dr.

Ok good news / bad news.

Been working on the merc 800 just so I can keep the boat on the water. Have plans the end of the month to take the boat up to the family reunion to take the kids out and play. So the 800 is working well. Water pressure seems low. connected the water line to the middle back port at the top of the jugs, cylinder block cover, ( showing only 8 to 10 psi ) witch I believe may be the wrong placement. Should water pressure pickup point be instead hooked up at the lower portion of the exhaust manifold cover? I purchased a new impeller, but current one seems to be pissing fine all the time.

Ok now back to the 115.

Good news.
Got her torn apart. Everything came apart pretty easy. Previous owner replaced gaskets 2 years ago. after talking to him I found that he used ant seize on the inner exhaust plate bolts instead of the epoxy or red loctite. Everything came apart pretty easy, but everything seemed still snug. Found that the gasket walked or squeezed out a bit on number 4 and looked the same on number 2, while number 2 ended up failing fully where number 4 looked like it was about to give out. So no cracks and I found where the water was coming from! YAAAAAA!

Now the bad news.
This leads me to my own dilemma on what to do next. number 1 piston has some pretty good scoring, ring shows score as well. ring may be pinched in the piston in this spot. surprised I still have 115psi compression. but this explains the lower compression in this cylinder. little bit of scoring in number 2. 3 and 4 looked good on the exhaust side, to be honest I didnt even look at 5 and 6. I may want to look at 6 as it was another cylinder that was a little bit lower on the compression side.

Now the motor is tore down this far. What to do?
Options
1, get a gasket kit for the exhaust, put it all back together and run it.
2, get a gasket kit for the exhaust, put it all back together and sell it, then look for another motor.
3, Get an entire gasket kit, needed pistons and rings (if available) Hone or bore, then keep and run it.

I already have $1500 in it. I would be leaning toward options 2 or 3 as I would be upset if I just ran it and it destroyed itself soon. What do you all think?

#1. Some motors run very well with low compression cylinders. Maybe a little hard starting but as long as you don't mind and the junk is cleaned out number 1 would work imho.
#3. If you already have a spare that you are running, have fun over the summer. Speed and horsepower shouldn't govern having fun with the kids.
#2. Completely out of the question. I would never do that unless you fully disclose the issue with the buyer. Who knows someone might be looking for a project. or parts.

Brought a pic for ya all.

Top piston (left one in photo) has quite the gouge. Picture makes it look way larger than looking at it by eye. but maybe thats because I dont have my readers on my face when im looking at it in the garage. Number 2 has some gouging and looks like a brass color on the skirt when rotating the motor to see it. The other pistons look pretty darn good from what I can see through the exhaust ports.

That is shot. Doesn't matter what you do with the outsides, it's never gonna run right or be right without a teardown and repairs. The upper piston in the pic is really really bad, it's more-than-likely to need a bore job and oversized piston.

The one below isn't quite there yet, maybe that cyl can be ball-honed and then see if there's any significant scoring. I'd say that piston will definitely need replacing, though.

BTW, on your 80 hp, they don't have a lot of water pressure at idle, if you're getting 10 psi or so while underway that's probably as good as it's gonna get. Is the impeller brand new? If not, that's a good reason for a lower reading. You'll probably never get above 12 psi at any rate. Long as the telltale has a good stream and the exhaust manifold cover isn't getting hot after a hard run, you should be OK.

I want to agree that it is shot, I will not argue with you or anyone on that just by looking at the picture. But I still have 115psi compression in number 1 with no water intrusion on this cylinder. Also other than a little rough idle (I believe mostly from the water in number 2) she still runs like a raped ape! I am hoping to hear some options here because If new pistons and rings are not available then the only choice I see is to purchase an exhaust gasket kit, reassemble, and just go run it till she dies. I cant see her being an anchor or wall hanger. Maybe Dr_Frankenmerc has some options that will keep this 1979.5 115 walking the lakes here and terrorizing the towns folk for a long time.

On the Merc 80, I really dont remember if I replaced the impeller in it when I purchased it. I feel like I know better and did, but so much was going on around that time I may have neglected this step. So I am not positive. This is why I purchased a new one. The old one is making the 80 piss constant and strong at all speeds currently. I just read somewhere that when hooking up the water pressure line, that you go with the lower point at the motor as this is the closest to the impeller and will give a most accurate and a higher reading, and that the point near the top of the cylinders would be for a water temp gauge. After running again, I am not getting 10psi at WOT, the motor is plenty deep, actually maybe 1/2 to 1 inch too deep after looking at the anti-ventilation plates location in the water with buddy driving it on plane. Im lucky to be getting 8 psi showing on the Sierra gauge I just installed. The exhaust plate is not getting hot to the touch nor is the water from the pisser anything but slightly warmer.

Dr_Frankenmerc wrote: The 1979.5 115 is an inline-6,...Not related to the posted image by Jim.
Ideally, replacement of gasket 14 "might" resolve the issue, but I strongly encourage replacing 14, 17, & 19, plus insuring the "water Jacket Cover" #13, is not warped, or corroded,etc.
The "Exhaust Manifold Baffle Plate" #18, also has to be carefully inspected for corrosion pits/holes, too.
This is not an easy task, and short-cuts will commonly result in the water intrusion rearing its ugly head again.
If you see cracks between the exhaust ports, forget spending money on the beast.
doc

Doc, Can you share the link where you get this parts diagram please? I am having difficulty finding this as I dont find anything saying 1979 1/2 in a search. the 79s show using a distributer, while mine has the coil packs.

If I am not mistaken the 79.5 is the first without the distributor.
Am I correct on that?
Are internal parts, cylinder block, pistons, interchangeable between these years if I found a donor motor?
If so, what years are interchangeable for cylinder block and or pistons. I assume that blocks may have a larger gap in years that would be the same than the pistons, but may not.

Very typical failure on an inline 6. any adi 1150 motor will be same block internals are the same dist or adi except there are hi dome and low dome pistons its the height of the deflector. all adi are low dome most dist models are hi dome 78 was when this change crept in. You also have the powerport hole in the piston and corresponding pp in block so youll need parts from a 1150 adi to be safe.
your pistons are toast push on the rings likey no spring back and they are stuck in the ring lands due to the smeared piston skirt. Sure you can run it but cyl wear will be fast your idle quality will deteriorate till its impractical and the block will def need a rebore. Right now a cleanup, hone and piston might save the day. so either that or id just look for a powerhead to swap on plenty come up on ebay or facebook. Unfortunately you now know compression numbers dont tell full story only a look through the ports do , exhaust side is hot so usually where damage appears first. I've got a whole right up here on correcting the weakness of the inner water jacket cover.

These should be the parts diagrams for your ADI 115, it's the first serial number group that shows the 6-coil ignition system:

www.marineengine.com/parts/mercury-outboard-parts/115-6-cyl/5430527-thru-5738890-usa

Verify with your serial number (plate is riveted to the transom bracket).

HTH............ed