15hp '82 johnson loses power/dies at high throttle on the water

[RedbaranTX]

Motor: 1982 15 Hp Johnson
Boat: 14ft Aluminum flat bottom

Symptom: Loses power and dies when running at high throttle on the water.

Facts: -will run on water at lower speeds all day
-will get up on plane for a few seconds at most, sometimes
-starts and idles easily and smoothly
-amateur compresstion test shows 62/58
-motor runs smooth at high speeds out of gear
-carb is clean
-performed de-carb as described in top-secret files
-new fuel lines
-good fuel
-new plugs
-re-built fuel pump
-**possible spun prop

** I have a new prop on order, could a bad one cause this?

I'm determined to get this thing going before I miss all of duck season!!!
Thanks for the help!

 

[Rick.]

Re: 15hp '82 johnson loses power/dies at high throttle on the water

Well you've done a lot of checking things out. If you rpm goes up but the boat bogs then I would expect a spun prop. What your describing sounds to me like a spark problem. These motors start and run like a dream on one cylinder. They simply lack power if one is not functioning. I would suggest a spark gap tester should be your next investment. Make sure to check it when the engine is warmed up good and giving you symptoms you describe. I have the exact same motor and love it. Your compression is low, but even. That could cause you power problems as well but I would check your spark first. Best of luck.

 

[bktheking]

Re: 15hp '82 johnson loses power/dies at high throttle on the water

If the compression is accurate it's pretty low, that could be the issue.

 

[RedbaranTX]

Re: 15hp '82 johnson loses power/dies at high throttle on the water

Thanks for the replies.

Far as the spun prop, the motor does not rev when it has the issue, but it does kinda flutter up and down; more down than up if you know what I mean. It does give the sensation of a spun prop for just a second before it dies down; it seams to lose push and then the engine slows.

I'll run to O'Reily and see about a spark gap tester. I was just having my trusting neighbor hold the wire for me, but they were both jumping pretty good then (both wires). I guess I'll know what I'm looking for when I see it... Only problem I foresee is that I can run the snot out of it in neutral at home, it doesn't have the problems until it's on the water in gear, so it'll be hard or impossible to test it while it's having the issue.


Thinking ahead, where do I start to remedy the low compression issue? I checked before and after the de-carb and it made no difference in the results.

 

[steelespike]

Re: 15hp '82 johnson loses power/dies at high throttle on the water

After it dies can you crank it?Will it start?Is it pumping water?
Carefully feel the head.After running it should be too hot to hold your hand on but not so hot it burns you.Also water flicked on the head should not sizzle.

 

[RedbaranTX]

Re: 15hp '82 johnson loses power/dies at high throttle on the water

STEELESPIKE:

It cranks right back up, no problem.

Cooling system is working great, it pees like no other and never gets too hot. I re-worked all that too.

I like your thinking though!

 

[bktheking]

Re: 15hp '82 johnson loses power/dies at high throttle on the water

If the compression is that low i'd be pulling the head to inspect for possible cylinder damage. The motor could be tired, need new piston's and rings due to it being a 28 year old motor and a high revving one at that. Doubt the headgasket is gone, numbers are too close. I did a headgasket on an early 80's 15 a month ago, both cylinders tested at 100+ when I was done the job.

 

[RedbaranTX]

Re: 15hp '82 johnson loses power/dies at high throttle on the water

BKTHEKING:

That's sounding promissing. I've never broken into the block of an outboard, but I ain't scared! If you know, would you mind telling me about how much that re-work cost you/them in parts? Not that mine need the same, but just to get an idea...

Also, are we talking any special tools or gadgets? I usually just dive into these things. I figure if I can't get it back I'll take it in, and at least I don't have to pay someone else to take it apart...

 

[bktheking]

Re: 15hp '82 johnson loses power/dies at high throttle on the water

The head can come off without pulling the powerhead, torquing the bottom 2 bolts is tricky, you can't get in with a torque wrench. I just try and gauge the tightness with a wrench. A head gasket is like 12 bucks. A complete rebuild kit can be purchased on ebay for a couple hundred bucks, they aren't difficult to rebuild unless you get into boring the cylinders. Like I said, the compression is low and may be effecting your powerband. Before going to all this trouble, try another compression gauge before anything else. You are pulling both plugs when testing, correct?

 

[RedbaranTX]

Re: 15hp '82 johnson loses power/dies at high throttle on the water

BKTHEKING:

I did pull both plugs while testing. I didn't warm the engine up first, is that necessary (ambient temp here is 80)? I'll go to a different Auto store to rent one next time, but I think I may do the head gasket too, it sounds fun.

I also just bought and used a spark gap tester. I couldn't warm the engine tonight, but I got 1/2 and 3/8 inch. How's that sound?

 

[bktheking]

Re: 15hp '82 johnson loses power/dies at high throttle on the water

Sounds good to me. You shouldn't need to warm it up, a cold motor should still put up high numbers.

 

[RedbaranTX]

Re: 15hp '82 johnson loses power/dies at high throttle on the water

I'll try the compression with a new guage and probably go ahead and swap the head gasket too. THe pistons and rings are a little more expensive so I'll wait to see if they have to be done or not. I'll be sure to look around while I'm in there.

I'll check back in when that's done, but please let me know if you think of anything else in the meantime, or if there is something specific I should look for in/under the head while it's off.

Thanks a lot for the help!

 

[RedbaranTX]

Re: 15hp '82 johnson loses power/dies at high throttle on the water

I couldn't help it, I had to take the sucker apart! I don't know if you can tell anything from pictures but here they are, shrunk for the post. The cylinders didn't have any apparent cracks or scrapes or anything and the visible parts of the rings appear to be in order.

I couldn't see anything obvious, but I also don't know what I'm looking for. They did make a nice clean phlumping kind of noise when I turned the flywheel. New gaskets as soon as I can find them, hopefully in town.

I'll clean up the pistons a little if the stuff comes off easy, and obviously clean off the face for the new gaskets. Anything else I should do?

 

[Rick.]

Re: 15hp '82 johnson loses power/dies at high throttle on the water

Can't tell much from the pictures except that your decarb. didn't do much for the pistons. I would suggest you do it again. Make certain your head is good and flat before you bolt it back on. Put some 120 grit sand paper on a piece of glass and go over it in figure eight strokes until all mating surfaces are nicely and evenly buffed. By the look of your pistons you could have stuck rings. Seafoam should loosen them up. Rick.

 

[raczekp1]

Re: 15hp '82 johnson loses power/dies at high throttle on the water

in august i did flattening of head from mine 9.9 in auto workshop. i payed 5 dolars.it was wraped quite a lot.they had to take of about 0.5 mm of head surface

 

[RedbaranTX]

Re: 15hp '82 johnson loses power/dies at high throttle on the water

Thanks guys. I've got the sandpaper, got the seals. long night ahead of me now. Anyone with a manual or prior knowledge know what torque I'm to use?

 

[bktheking]

Re: 15hp '82 johnson loses power/dies at high throttle on the water

 

[RedbaranTX]

Re: 15hp '82 johnson loses power/dies at high throttle on the water

Can't thank you enough Mr. King

 

[RedbaranTX]

Re: 15hp '82 johnson loses power/dies at high throttle on the water

THIS IS NOT FUN ANYMORE!!!

I changed the head and water cover gaskets. I followed the guidance given in the thread, even the sandpaper.

I performed another de-carb since the pistons showed the last one was inneffective. I let the Seafoam sit in the cylinders overnight this time. I changed the spark plugs again after clearing it out.

Compression went from 62 & 58 to 70 & 60.

Took the boat out a few minutes ago and it did the same thing as before. Runs just fine at low throttle but loses power at higher.

I'm out of ideas, what you guys got? I'm running out of duck season and tired of walking through mud. I want to float over it!

 

[kfa4303]

Re: 15hp '82 johnson loses power/dies at high throttle on the water

I'm thinking it may be a fuel delivery issue. (Just a guess) Is the fuel pump working well? Perhaps something went wrong while rebuilding it. I've heard they can be tricky. Does the bulb on your tank get firm when primed? I had a motor that did a similar thing. Ultimately turned out to be poor fuel delivery.
After so many carb rebuilds, that's probably not the issue, and you say it starts easily, so the spark is probably ok, and it is colling well, has decent compression, so about the only thing left is fuel (right?) :/ You can try disconeecting the hose going from the pump to the carb and turn the motor over a few times. Gas should get pumped out of the hose, if t doesn't, then I'd say the fuel pump was bad, or maybe there is a clogged line. Just some thoughts. Stick with it. You'll get it.