Overheated 1980 25hp Johnson

[megacaver]

All,

I have read a bit on this great forum and now need some advice.

I have a 1980 25hp Johnson that I am trying to determine if its worth the time and money of saving. I've had the motor since 1988 and it has run very well and strong for me despite my shamefully lacking maintenance efforts. It set unused from about 1995 until 2000 when I rebuilt the carb, changed fuel lines and lower fluid. It ran fine the several times I took it out in 2001 but noticed milky lower unit fluid so I set upon finding the leak and in the same stroke change the very aged impeller though water was flowing fine at the time. On the next trip out (very cold 15deg) it was pumping water strongly at the dock but about 10 min into a run at full speed the engine seemed to start slowing down/bogging down. As it slowed down I noticed smoke from under the hood and hit the kill switch. It seemed hot to me and was sure at the time I must have screwed up the pump install and fryed the engine. I intended to get into it and see what was wrong ASAP....as things go 8 years later I am now getting into it.

After reading other posts I purchased a can of deep creep and a compression tester. Removed both plugs, sprayed with deep creep, pulled then engine through (rope start) and let it sit a while. Tested the compression and was getting 35psi on both cylinders. I then pulled the head to check the gasket and cylinders. I'm unsure what a blown gasket would look like and I need to scrape it off and check head for flatness. Bottom cylinder was very smooth all the way around, top cylinder has an area of roughness at 7 o'clock that I can definitely feel is not smooth but doesn't really snag a fingernail. I have included pictures.

Should I buy a new gasket, hone the head and see what my compression is? Should I not bother and buy another similar used omc engine and use mine for parts? Should I tear farther into the scuffed cylinder for more information?

I and my 6 year old (who wants dad to get his boat working) thank you for your time,
James

 

[steelespike]

Re: Overheated 1980 25hp Johnson

Did you pull it repeatedly to get your reading.35 is really low but its good its
even.Being so even if the gasket was bad it would seem it would be in an area shared by both cylinders.A spot that looks like the compression was blowing through it.It sure looks like the piston is probably at the least scuffed.The silvery specs are troubling as well.Looks to me like a tear down is in order.Then there is the question why it over heated with a brand new impeller.Sometimes the impeller hub will let go.
Or perhaps you didn't get the water tube in right and it got out of place.
I'm not into tear downs anymore I'd be more inclined to look for a better motor.
perhaps newer technology lighter or electric start more hp etc.

 

[Johnny-X]

Re: Overheated 1980 25hp Johnson

Looks to be time for a rebuild if your planning on keeping it

 

[R.Johnson]

Re: Overheated 1980 25hp Johnson

That gasket is certainly blown, thus the equal low compression. It's been hot but the paint doea not appear discolored. That streak, or smear on the cylinder wall is aluminum from the piston. You could replace the head gasket, and it could run a long time, but that is hard to say.

 

[jpriak]

Re: Overheated 1980 25hp Johnson

Those scratches in that 3 pic say it all, thats blown. I don't think a motor will run at 35 psi anyway, your compression should be more like 100-120psi. It's time for a overhaul.

 

[bktheking]

Re: Overheated 1980 25hp Johnson

I'd hone it and see how it turns out before anything.

 

[megacaver]

Re: Overheated 1980 25hp Johnson

I cleaned up the head tonight and laid it on a piece of glass and it did not seem to be warped. So I put a piece of emery paper under it and honed it; this cleaned up the mating surface pretty quick. Have attached a picture after honing. I will order a new head gasket, put it back together and test the compression. That should tell me something if the psi goes back up and is similar between cylinders or that one scuffed up top is falling off from the other. Will post when I have new compression numbers.

 

[R.Johnson]

Re: Overheated 1980 25hp Johnson

The low compression is the result of that head gasket. The scuff on the cylinder wall is never a good sign. In fact, if you could see it the piston will look worse. I would run it, see how it sound's, and take another compression test. Let us know what you find.

 

[megacaver]

Re: Overheated 1980 25hp Johnson

I have the new head gasket now and a used service manual for my year 25hp Johnson. Would like to put it back together tonight but have a quick question. In reading the manual it says to use gasket seal unless the gasket states otherwise. I have the correct OMC gasket but it doesn't say anything on it. From what I have read on the forum no seal compound should be used just clean the block and head side very well; I believe that to be correct also. My question relates to the fact that the gasket I have, and all others for this model I have seen pictures of, is metal covered with a small amount of rubbery compound EXCEPT for the sliver directly between the cylinders facing the piston side. Won't this allow it to leak between the two cylinders? I just want to make sure I am doing it right as the compression check after this will mean a lot as to the next step I take. Again I don't think the gasket is defective as all others I've seen have the same metal only area on that one side.

Much thanks to all.

 

[megacaver]

Re: Overheated 1980 25hp Johnson

Installed the new gasket and re-tested compression. 5 pulls on the rope for each cylinder trying to be consistent gave me 105 psi on the bottom cylinder and a little higher 107ish on the upper cylinder, pictures attached. The upper being the one with the scuffed wall back in pic #3 in the first post.

So advice as to what I should do next?

Should I worry about addressing the rings aka the skuffed cylinder? At a minimum I would think I need to rebuild the carb as it has been sitting along time. Rebuild the water pump as it may have been the reason for the overheating. New thermostat as it look like crap and flush the waterways. New coils as they look a bit melted where they contacted the overheated cylinder head (should I test the spark before replacing?). Decarb if I get it running.

Any advice for my next step is appreciated.

 

[psteurer]

Re: Overheated 1980 25hp Johnson

You have a good plan of attack described in your last post. For me the next step would be the water pump since that is what caused the problem to begin with. Pulling the thermostat and cleaning the water passages would also be good as you say. Checking for spark is a good idea. Since your compression numbers now are acceptable, why don't you just put some fuel in it and see if it starts.

 

[megacaver]

Re: Overheated 1980 25hp Johnson

Wanted to add an update on this. After replacing the head gasket and getting good compression cold, I replaced the coils, spark plugs, new carb kit, new fuel pump kit, new fuel lines, new thermostat, and new starter rope. Tested spark and it was good. Mixed up a batch of new fuel, set it in water and fired it up. It was not that hard to start but I immediately saw that no water was streaming and a lot was coming out the exhaust holes. Shut it off and unbolted the lower unit. After using a flashlight to look up inside the exhaust housing I noticed the ample number of places the water tube could go other then the port to the power head. Looking at the copper water tube I confirmed the mystery was solved as it had a bright spot on the outside where it was rubbing against the outside of the water port. I had missed putting tubing into the port as I reattached the lower unit. Using the flashlight and a compact mirror I carefully made sure the tube went into the port and bolted the LU back one. Back in the tank and fired it up again. This time lots of water out the stream! Last night I ran it in the tank for about 20 minutes, took the plugs out and tested compression again but hot and got 114/110. Temperature at the water jacket top stayed between 135 and 145 once it was warmed up. So bottom line I got in too big a hurry installing the LU, then didn't test it, and compounded that by being in too big a hurry to notice the steam wasn't present that next morning now many years ago and blew the head gasket. Not the best sequence of decisions.

Thanks for all the help on this. I will start a new thread for any other questions as I have the kits already for the impeller and to reseal the LU. The "new" one in there has now been setting idle for years and I don't trust it and want to make sure the LU is water tight. A good de-carb is on the list too.

Thanks Again.

 

[jasper60103]

Re: Overheated 1980 25hp Johnson

Glad to hear you got her running again. Also, it sounds like you learned a lot from your experience. I have the same motor, j25 '81. I haven't done much work on it. I changed the lower unit lube and water pump. If I ever run into problems I hope you don't mind if I ask a question or two. Thanks again for the post.