15 hp sudden compression loss

[MushCreek]

I picked up (literally, out of the garbage) a 2000 Johnson 15 the other day, and it has virtually no compression. The PO said it was running fine, made a 'pop' noise, and died. So far, I've pulled the head, expecting to see destruction, but everything looks perfect. The head gasket looks OK also, or at least not bad enough to lose all compression. What surprised me was to see a small hole in each cylinder above the ports on the exhaust side. It definitely belongs there; What is it's purpose? Could something have blown in there to take away the compression? I also have no spark, but that could be a safety switch or something that I haven't found yet. If I can't sort it out, I'll just part it out, but it's worth a shot to fix it.

 

[tashasdaddy]

Re: 15 hp sudden compression loss

remove exhaust cover, and look at the rings.

 

[bktheking]

Re: 15 hp sudden compression loss

Part it out (Nice find BTW)? Ship it to me. Anyways, rings or headgasket, having no compression on both is a rarety and you'd expect to see damage. I vote head gasket, check the head for warpage on a pane of glass. Those are the big block, not the ones I'm used to working on, if they look like machined holes chances are they are part of the construction of the block.

 

[kenmyfam]

Re: 15 hp sudden compression loss

Could well have blown a ring or 2. Check the head while it is off. Look at the rings through the exhaust cover.
What numbers would be "virtually no compression"??

 

[MushCreek]

Re: 15 hp sudden compression loss

I should have put a compression tester on it before I took it apart, but mine is out on loan to a friend. It feels like there are no spark plugs in it- I mean NO compression. I'll keep digging.....

 

[mikesea]

Re: 15 hp sudden compression loss

ARE YOU SURE PISTONS R GOIN UP AND DOWN,POSSBLE BROKE CRANK??JUST A POSSIBILITY,EITHER WAY,ITS A GREAT FIND IF IN OTHERWISE GOOD SHAPE,IF ITS A PISTON/CYL.THING ,REBUILD IT,EASY ,NOT EXPENSIVE ,WHEN DONE iD BET ITS WORTH 1000 TO 1500

 

[MushCreek]

Re: 15 hp sudden compression loss

Okay- Now we're getting somewhere. The head is warped- a lot. I would say at least .015 to .020. Can I deck that much off? I'm a tool maker, so decking it is an easy job, but that seems like a lot to take off. On close inspection, the gasket looks discolored between the cylinders- that's probably where the compression was leaking. I took off the exhaust port, and it looks great. Rings look perfect, and you can still see cross-hatch in the cylinders. No scoring anywhere. BTW- there are definitely holes from each cylinder into the exhaust. I would say about 1/8" in diameter, and cleanly machined. I've never seen anything like that.

 

[kenmyfam]

Re: 15 hp sudden compression loss

Okay- Now we're getting somewhere. The head is warped- a lot. I would say at least .015 to .020. Can I deck that much off? I'm a tool maker, so decking it is an easy job, but that seems like a lot to take off. On close inspection, the gasket looks discolored between the cylinders- that's probably where the compression was leaking. I took off the exhaust port, and it looks great. Rings look perfect, and you can still see cross-hatch in the cylinders. No scoring anywhere. BTW- there are definitely holes from each cylinder into the exhaust. I would say about 1/8" in diameter, and cleanly machined. I've never seen anything like that.

You need to take off whatever is required to get it flat again. Take a measurement before you start in multiple places (you are a toolmaker so I don't need to explain how) and when it is cleaned up and flat take the measurements again and compare. You don't want to start "bumping" the spark plugs so if necessary put a double gasket on it.

 

[torbjorn]

Re: 15 hp sudden compression loss

Okay- Now we're getting somewhere. The head is warped- a lot. I would say at least .015 to .020. Can I deck that much off? I'm a tool maker, so decking it is an easy job, but that seems like a lot to take off. On close inspection, the gasket looks discolored between the cylinders- that's probably where the compression was leaking. I took off the exhaust port, and it looks great. Rings look perfect, and you can still see cross-hatch in the cylinders. No scoring anywhere. BTW- there are definitely holes from each cylinder into the exhaust. I would say about 1/8" in diameter, and cleanly machined. I've never seen anything like that.

Our '81, I milled the head .030", milled the block .010", accelerates like a beast. Run at least pump 93 if you mill .040".

 

[torbjorn]

Re: 15 hp sudden compression loss

You need to take off whatever is required to get it flat again. Take a measurement before you start in multiple places (you are a toolmaker so I don't need to explain how) and when it is cleaned up and flat take the measurements again and compare. You don't want to start "bumping" the spark plugs so if necessary put a double gasket on it.

Standard head gasket is more than fine if you mill .040".

 

[MushCreek]

Re: 15 hp sudden compression loss

Once I got things cleaned up, the head looks like it got HOT. It's hard to tell with black paint, but that's my guess. The motor is a salty dog, so I'll bet the pump is shot, or the passages are clogged. Luckily enough, all the bolts are coming out without any drama (unlike my '83 35 hp) so it's worth salvaging. Anyone know how these late model 15's are? Do they have enough grunt to put on a little hydro to play with? Or I might just sell it and put the money into my Whaler project.

 

[bktheking]

Re: 15 hp sudden compression loss

They actually race with 9.9's/15's on hydro's but they are race prepped motors, i'm sure in stock form it would do ok. If you don't need it, fix it and sell it, or ship it to me (X2time) LOL

 

[MushCreek]

Re: 15 hp sudden compression loss

Just an update: I decked the head, taking off about .030 total, but only about .020 in the actual combustion area. It was warped both ways, so it was bowl shaped. I threw it back on with the old gasket, and pulled 110 lbs on both cylinders, cold and dry. It still pulls a LOT easier than my '56 15 hp Johnson, and that only blows about 90 psi. Maybe those little holes are some kind of compression release? Anyhow- I guess I'll invest in the gaskets, and continue on with it. There's definitely no spark- any ideas for a guy that was raised on the old magnetos? I'd like to keep my investment down until I know the motor is actually worth it!

 

[kenmyfam]

Re: 15 hp sudden compression loss

Standard head gasket is more than fine if you mill .040".

Agree. It was posted as a warning only. Ever seen one that someone took over 1/8" off ??? Not a pretty sight. Also who knows if it has been machined down before this time ????
Hence the warning.

 

[boobie]

Re: 15 hp sudden compression loss

Those little holes in the cylinders were put there to make the mtr idle better. You have what OMC called a big bore 15 hp when they first came out with it in 1993 and there were some "shakey" probs with the first mtrs. The holes seemed to help the idle problem.

 

[MushCreek]

Re: 15 hp sudden compression loss

I used to work in a shop that rebuilt marine engines- mostly jetskis, but we did outboard work, too. On the head, there's a very slight step in the combustion chamber- about .015 or so. We used to figure we were OK to remove that step- any more, and we threw out the head. This one was just at that step when I finished. I'm sure it was never decked before.

 

[MushCreek]

Re: 15 hp sudden compression loss- Update!

Re: 15 hp sudden compression loss- Update!

I fired up the little 15 today, it runs pretty good! In all, I decked the head, cleaned out a lot of corrosion, replaced the exhaust cover gaskets, and cleaned the carb. The mysterious lack of spark was a bad safety switch. I just went around it- sorry, Ralph Nader. The motor started right up, and runs well. It pumps water like crazy, so I don't know what warped the head. It did look like it had gotten hot at some point. I think someone took the thermostat out, since it pumps water all the time. Or does it just show out the tell-tale all the time? At any rate, it ran nice and cool. Thanks to everyone for all your advice!