15 hp - no compression on one side

[bison716]

Re: 15 hp - no compression on one side

Here is a picture of the head. I hope you can see the notch broken out.

 

[raybow1]

Re: 15 hp - no compression on one side

Wow! Does it look like it has carbon burns leading out of the chamber where the failure is? It is hard to tell from the pics but looks clean right in that area. I wonder if it was a flaw in the head that finally gave or was it electrolosis that ate it?

 

[torbjorn]

Re: 15 hp - no compression on one side

Bought a used '93 - 15 hp Johnson, supposed to be low hours and hardly used. It was a sight unseen private deal and I got taken to the cleaners. Got the motor home, besides being very dirty it has no compression on the bottom cylinder. Called the seller back and he swears it ran great last time he used it but it has been sitting for 2 years since being run. OK, I'm a big boy and it was sold as is. I've rebuilt sled engines before but they have removable cylinders. My question is if you need to remove the entire power head out of the chassis or can you strip it down as is. Looks like you can remove the lower engine covers and get at all the bolts. Not quite sure how to proceed. Any advise would be appreciated. Thanks.

I rebuild 15s regularly, have 1981 and 1985 models myself. Pull the powerhead and rebuild it, could have a hole in the bottom piston. At the very least the rings are completely stuck or broken. First, get an OMC service manual from ebay, any year 1977-1993 will work. You can get all necessary parts from Seaway Marine in Seattle at good prices. You could pull the head first, as someone advised, to see what the problem is.

Some advice: the 15 is unusual, was rated 15 hp at 6500 RPM with normal operating range 6000-7000 RPM instead of 5000-6000. Run 25:1 oil, not 50:1 (unless you're trolling or running a lot a low speed), use only Evinrude, Yamaha, Mercury, Suzuki, or Castrol oil, TCW-3.

Good compression is 120-130 psi. I would rebuild any motor with compression as low as 100 psi, because blow-by is surely occuring and you're losing a lot of acceleration.

 

[jbjennings]

Re: 15 hp - no compression on one side

Here is a picture of the head. I hope you can see the notch broken out.

That is really weird!!! I have never seen anything like that. The water jackets and entire head look corrosion free and the head looks darned clean. I do believe that chip may even be big enough to give zero compression, which I doubted at first. I just can't imagine what could cause that to happen other than a foreign object in the cylinder. And you see no evidence of that on the cylinder walls as far as gouges or scratches, right? The piston looks really nice?
I just worry something else might be hiding to produce 0 compression. When you turn the flywheel with the head off, do the rings seem to be nice and tight against the cylinder walls?
You might try calling some of the salvage sites like laingsoutboards.com or some of the others to try to get a better deal on that head rather than buying a new one, since they pretty much should last forever.

It does appear that you've found your problem, though. If all your cylinders look really nice, I believe that motor will run like new when you get your compression up. Make absolutely certain you use a torque wrench and follow the correct tightening pattern when you install the head. Clean up all your bolt holes with a tap and don't use any gasket sealant on the head gasket.

I'm really interested in hearing your compression results with a new gasket and good head.

Makin' progress.....
JBJ

 

[tx1961whaler]

Re: 15 hp - no compression on one side

That looks like a casting failure. Why? I can see no evidence that there has been anything bouncing around on the head because the carbon looks undisturbed. The plug looks to be intact and a perfect tan.
That said, I'd get a different head.

 

[Rick.]

Re: 15 hp - no compression on one side

Agree with above. A new/used head would be where I would go now. Best of luck. Rick.

 

[1946Zephyr]

Re: 15 hp - no compression on one side

That head looks like it may have been damaged, from a previous removal.

 

[tx1961whaler]

Re: 15 hp - no compression on one side

That head looks like it may have been damaged, from a previous removal.

That crossed my mind, but I don't see the normal telltale signs of the "flat bladed screwdriver being driven in by a hammer" on the outside edge of the head.
Anyway, it's broken. I'd try to find a used head, or get a new one if money isn't an issue.

 

[bison716]

Re: 15 hp - no compression on one side

My nearest dealer is 80 miles away and they don't have a head in stock. One of my coffee buds reminded me there is a pretty good machine shop in this area not too far away and maybe they can weld it and mill it straight. Will talk to them next week. It's worth a try. As far as the other questions, I checked the cylinder walls again today and there is not a mark on either one. Smooth as glass. I'll see what I can find out this next week and keep you posted. Thanks.

 

[bktheking]

Re: 15 hp - no compression on one side

Make an offer

http://cgi.ebay.com/ebaymotors/JOHN...7594134QQptZBoatQ5fPartsQ5fAccessoriesQ5fGear

 

[bison716]

Re: 15 hp - no compression on one side

Make an offer

http://cgi.ebay.com/ebaymotors/JOHN...7594134QQptZBoatQ5fPartsQ5fAccessoriesQ5fGear

Wow, thank you. How the heck did you find it? I spent a good hour searching on E-bay and didn't find this. I have already sent the seller a request for a freight quote and will track this item. It doesn't run out for 19 days, so I have time to try and get my head repaired in the meantime.

 

[71Windsor]

Re: 15 hp - no compression on one side

If you have a descent machine shop I would let them give it a shot. But always double check even a professionals work. No one is perfect. Make sure its an even surface when you get it back. I take a lot of stuff to machine shops to be repaired and on rare occasions have had it come back just a little off. But with these types of mating surfaces you cannot settle for a little off. I mainly work with dirt bike heads but same principle.

 

[torbjorn]

Re: 15 hp - no compression on one side

My nearest dealer is 80 miles away and they don't have a head in stock. One of my coffee buds reminded me there is a pretty good machine shop in this area not too far away and maybe they can weld it and mill it straight. Will talk to them next week. It's worth a try. As far as the other questions, I checked the cylinder walls again today and there is not a mark on either one. Smooth as glass. I'll see what I can find out this next week and keep you posted. Thanks.

Don't weld. Heads are cheap on ebay.

 

[torbjorn]

Re: 15 hp - no compression on one side

news to me!!:redface:

What law of physics would that be?

 

[tootallofwa]

Re: 15 hp - no compression on one side

My brother had a T-Bird w/429 had a grove between cylinders 1/4"x1/4". guy that sold it to him used JB Weld to fix it the first time. Didn't last too long.

It was groovy!!!

Weird stuff happens...

 

[bktheking]

Re: 15 hp - no compression on one side

Wow, thank you. How the heck did you find it? I spent a good hour searching on E-bay and didn't find this. I have already sent the seller a request for a freight quote and will track this item. It doesn't run out for 19 days, so I have time to try and get my head repaired in the meantime.

Magic I guess, I wouldn't weld it either, second vote on that one.

 

[bison716]

Re: 15 hp - no compression on one side

Had my son-in-law over today for Easter dinner. He is a farmer and a ticketed diesel mechanic and a boater to boot. He looked at the head, and has a good theory as to the cause of the break. He suggests the motor was not drained properly and then stored with the head down. There was likely a pocket of water formed at the bottom of the head and then froze, breaking out the water jacket at that point. That would explain why the broken area was very clean with no carbon marks. Could be right. Anyway, will try to get a replacement head and report back. Thanks again.

 

[bktheking]

Re: 15 hp - no compression on one side

Ya but that doesn't explain where the broken piece is, I think it was a factory defect and a piece was missing since day one, a piece of metal that size breaking off and ending up in the cylinder would have grenaded that motor, I think the gasket got weak and compression went out the window, that's my theory.

OR

Someone had this head off in the past, used a screwdriver to get the head seperated from the block and the screwdriver took that piece off the head.

 

[1946Zephyr]

Re: 15 hp - no compression on one side

Ya but that doesn't explain where the broken piece is, I think it was a factory defect and a piece was missing since day one, a piece of metal that size breaking off and ending up in the cylinder would have grenaded that motor, I think the gasket got weak and compression went out the window, that's my theory.

OR

Someone had this head off in the past, used a screwdriver to get the head seperated from the block and the screwdriver took that piece off the head.

Yea, that was my guess too. Oddly enough though, if there was a pocket of water that froze and broke that little piece off, it was darned lucky that it didn't end up in the cylinder. You might look around and see if it is lodged in the water jacket somewhere.

 

[Rick.]

Re: 15 hp - no compression on one side

Your s-i-l might be on to something. If it was that way from day one I don't think your motor would have ever run worth a dam and would have been diagnosed by the dealer. At least that is what I hope would have happened. Let us know once you get a new head on it. Rick.