Melted Pistons - 1981 115 HP Johnson

[JTKelleyiii]

Greetings!

I have noticed several posts lately about melted pistons so I thought I would share this and also ask some questions.

I was given a 1981 115 HP Johnson, J115TLCIH. Previous owner had no idea as to condition just wanted it gone. Has a nice Rapture prop on it though.

Right out of the gate I checked compression. Results as follows:

1 - 130 psig
2 - 10 psig
3 - 130 psig
4 - 120 psig

Pulled the heads and this is what I found:



The question I have is what would melt both pistons on one side? Based on what I have read on this site lately it seems that the biggest cause of this is an overheat condition. Water deflector on that side is not stopping flow. What else might cause an overheat on one side? Some kind of blockage in the thermostat assembly or water circuit elsewhere? Have pulled the thermostat off and see nothing grossly wrong.

Also, could some type of ignition issue with powerpack on one side cause this? This engine has a power pack on each side.

While the heads looked like they are scored there is very few areas you can actually feel. I also have discovered that all four cylinders on this motor have been bored out to .030 oversized already. What a bummer for somebody to put that kind of effort into the motor then fry two pistons.

Anyway, I am in the process of tearing it down. If crank and internals look good will bring to the machine shop and have it measured up and see if it is worth rebuilding (will require at minimum 2 new pistons, rings, etc.). If it is too much then I will save the prop and see if anybody wants the parts. Won't be out anything except the time tearing it down.

Any thoughts would be appreciated.

Thanks,

John

 

[Faztbullet]

Re: Melted Pistons - 1981 115 HP Johnson

I see two things that could cause this: 1) Someone has added a 140 pot belly and could be sucking water in that side can caused by overpressure of steam pockets, the exhaust ports are a little to clean. 2) the pack/s or timer base having a problem as they are tied together the bias on white wire. It looks more like preignition as dome and exhaust side are eroded and the likes of have both jets on that side lean from 2 separate carbs is slim....

 

[kmarine]

Re: Melted Pistons - 1981 115 HP Johnson

Perhapse I am wrong, however I was taught that if you bore an engine over and replace with larger pistons in a two stroke engine that you will need to re jet the carbs or you will run too lean and melt pistons. There is a possibility that the oil injection pump may also need calibration or have a blocked passage way. As soon as metal particles start breaking lose inside the engine the surfaces look like they were shot with a shotgun.

 

[Faztbullet]

Re: Melted Pistons - 1981 115 HP Johnson

I was taught that if you bore an engine over and replace with larger pistons in a two stroke engine that you will need to re jet the carbs or you will run too lean and melt pistons.

Only if over .030, they will run all-day at .020 on stock jets...

 

[emdsapmgr]

Re: Melted Pistons - 1981 115 HP Johnson

I looks like the rubber water diverters are correctly place, from what I can see in your pictures. That's good-and usually the first place to look in this situation. Since those probably did not contribute to an overheating problem, look elsewhere. But still, it can be a cooling issue. 1. I'd pull the head cover off the head on the damaged side. See what is inside the head in the cooling passages. You might find weeds, sand, etc 2. I'd replace both thermostats. If one sticks shut, it will overheat that side at idle. 3. There is a nylon spacer in the thermostat housing called the valve body. It should have two pinholes in it. They frequently plug up with dirt/sand over time. If one or both plug up, it will prevent hot water from causing the thermostat to open on one/both side(s) and can cause an overheat at idle. 4. Check the nylon pressure relief valves in the stat housing also. If they get heat-deformed they may not open properly when you go on plane and it can overheat at high rpm's. 5. Always possible it is a carb problem-pull the high speed jets out of both carb bowls that feed the damaged cyls. Visually inspect for debris in the jet. Usually you do not need to rejet after an overbore if it is up to .030 over. Any overbore larger than .030 and you should make some rejet modification on the high speed jets at least. I do see what appears to be a little debris between the top and bottom cyls. Those passages should look like the attache pic. You'll want to clean up your coolant passages around both cyls before re-assembly.

 

[JTKelleyiii]

Re: Melted Pistons - 1981 115 HP Johnson

I checked the part number for the "pot belly" and it is for a 140.

Also, the exhaust ports on all pistons are clean.

Why would they add the 140 exhaust manifold? To get more HP?

This is looking more and more like a parts motor candidate.

Thanks for all of the feedback. If anyone has any other thoughts let me know as I am always try to learn.

John

 

[JTKelleyiii]

Re: Melted Pistons - 1981 115 HP Johnson

I meant to say exhaust ports on all cylinders.

 

[HighTrim]

Re: Melted Pistons - 1981 115 HP Johnson

Unfortunatley alot of times when a motor blows, the owner rebuilds it and forgets to investigate into WHY it blew in the first place.

It is only a matter of time until it happens again. Rebuilding a powerhead wont fix most of the issues that get it into the rebuild stage in the first place. The reason I try to stay away from rebuilt motors myself. Unless I know the mechanic. Guys like emdsapmgr and faztbullet obviously know what they are doing, but there are numerous back yard mechanics out there that should not be rebuilding motors and charging for it.

 

[V153]

Re: Melted Pistons - 1981 115 HP Johnson

View attachment 176001There is a nylon spacer in the thermostat housing called the valve body. It should have two pinholes in it. They frequently plug up with dirt/sand over time. If one or both plug up, it will prevent hot water from causing the thermostat to open on one/both side(s) and can cause an overheat at idle. 4. Check the nylon pressure relief valves in the stat housing also. If they get heat-deformed they may not open properly when you go on plane and it can overheat at high rpm's

The thermostat housing/valve body is actually made out of Bakelite, not nylon. It is not at all uncommon for them to develop cracks'n leak 'tween the stats & relief valves. Yes the nylon relief valves can & do overheat/deform.

Good idea to go through the thermostats/housing/cover. Think the reason so many people blow it off is the sob(s) are so hard to get at?

 

[daselbee]

Re: Melted Pistons - 1981 115 HP Johnson

Am I missing something? What is all that red crap in between the cyls and the block? From his pics it looks like someone filled it with orange/red RTV.

I thought it should look exactly like EMD's block picture.

 

[Faztbullet]

Re: Melted Pistons - 1981 115 HP Johnson

Looks like the factory corrosion coating is peeling as itsa reddish brown color....

 

[JTKelleyiii]

Re: Melted Pistons - 1981 115 HP Johnson

It is some kind of adherent coating. See no signs of peeling. It is a red brown color for whatever reason.

 

[emdsapmgr]

Re: Melted Pistons - 1981 115 HP Johnson

Possible the last person to install the head gaskets used some type of sealer. Most factory head gaskets today do not require any sealant. Bakelite, now there is a word I have not heard of in years-I'm sure you are correct.