15hp low compression and then......small plastic pieces by theromostat

GreenHornet763

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1974-76 15hp johnson

So I did a compression check manual pull start (motor has electric start), 68 on both cylinder cold start. Thought had a bad carb because it woiuld literally take a minute on the electric start to start and run for about 5-10seconds then dies. I notice no water pumping out of the tail. Checked water pump and impeller seems decent, the water pump was smooth with no warping, i changed out with a new impeller. Still hard starting, changed new spark plugs, still hard starting. checked if both plugs have spark, both turned out nice and blue. Tonight i took off head and there seems to be no damage to cylinders and pistons, I pushed down on the piston to see if there's any play but both cylinders had none.

I was clueless, then i didnt see a thermostat which i assumed most newer johnson's would have it but not this model. When i opened the plate from the head to remove the thermostat, i notice one of the water channels were moist and other side was dry (seems to have a blockage or thermostat not opening). Upon further investigation, i notice plastic little pieces in front of thermostat. Conclusion would be there's a blockage somewhere and water is not spitting out of the tail. Maybe need new thermostat or clean the water tubes for blockage? Any tips or advice is great appreciated!

GH
 

kfa4303

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Re: 15hp low compression and then......small plastic pieces by theromostat

Hi GreeenHornet763. A blockage in the water passages can definitely impair water flow. May be from an old impeller or thermostat gasket/grommet that degraded or other debris and got sucked up into the power head. I would definitely unclog the passage and get a new thermostat kit. They're only about $15 and you can get them right here at iboats. Once you have the head gasket and head back on you can test your water flow by dropping the LU and hooking up a garden hose to the copper water uptake tube. you should see water coming out of the exhaust port. You can also check the water flow by removing the thermostat and slowly pouring water into the the thermostat housing. Again, you should see the water exit through the leg. Try some of these tricks and let us know what you find.
 

Jim311

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Re: 15hp low compression and then......small plastic pieces by theromostat

I imagine the low compression numbers are what's causing hard starting. Until that problem is fixed you may continue to have hard starts.
 

raczekp1

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Re: 15hp low compression and then......small plastic pieces by theromostat

68 its to low.to have good and strong motor should be 100 or more
maybe one ring just stack in piston grove,try to play with sea foam
i know if you pure some oil in cylinders and check once more compresion and wll increase it means bad rings

now im workimng with johnson 15 hp, 79'.i puled the head and in water chanel leading to termostat i found dry sand.
the head changed colour from white to browm,
 

Rick.

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Re: 15hp low compression and then......small plastic pieces by theromostat

68 is low but a different gauge might improve on that. At least your numbers are even. The plastic pieces could be from you water intake screen located down low in the leg. That's the only place I can imagine where plastic could come from. Put a little gas mix in a spray bottle and spray it into carb. as your cranking it and see if starts easier. To be frank I'm not all that sure your motor has a tell tale. For some reason I'm thinking the added the tell tale later like in 77 0r 78. I might be wrong about that though. You can separate the leg and flush the motor from the water tube and then back flush it from the stat opening.. Hopefully you can flush any debris out. Best of luck. Rick.
 

GreenHornet763

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Re: 15hp low compression and then......small plastic pieces by theromostat

alright, this past week i've been lurking this thread and made some updates.

1st- pulled thermostat and found it not opening while in boiling hot water, replaced the thermostat and gaskets etc...
started the motor, hard start still but runs for like 2 minutes then dies. Thinking it is overheating, no water out of tail still.

Since no water is coming out of tail, I took off lower unit and hooked up a small hose to the water cooper inlet, no water pumps out
and seems to be a blockage somewhere.

So this is where I'm stuck, would I have to take the whole block to clean the water channels or is there a easier way to cleaning
the gourmet water tube without taking the block off?

Thanks,
GH
 

Haffiman

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Re: 15hp low compression and then......small plastic pieces by theromostat

Lift off the power head and most likely you will find the upper watertube grommet blocking!
 

Solittle

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Re: 15hp low compression and then......small plastic pieces by theromostat

On the hard starting & 2 min until it dies - - have you checked fuel? Your symptoms could point to air in the fuel lines, crud in the carb, weak fuel pump, etc.
 

kfa4303

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Re: 15hp low compression and then......small plastic pieces by theromostat

Hi hornet. You can check the water passages by taking off the head gasket and/or the exhaust housing gasket. But before you do that, you may want to try removing the copper tube and flushing it out. If you're lucky, the blockage is only in the tube. It will also give you a chance to inspect the grommet per Haffman's suggestion. Just give it a firm tug and it should come out. Also try removing the thermostat and pouring some water into the the housing area. You should see water coming out of the exhaust port as you pour the water in. This will let you check for a blockage form the top down rather than from the bottom up like with garden hose method. If neither of those work, then you will need to take of the head gasket and/or exhaust plate to check the passages in the engine block itself.
 

GreenHornet763

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Re: 15hp low compression and then......small plastic pieces by theromostat

So I had time yesterday to touch this, been frustrated with this :(
I took off the head and poured water down the hole at the bottom on the block, water goes down
to the copper tube. I took off the thermostat and reinstalled the head w.o the theromstat. Hooked up
the garden hose and nothing, seems to have pressure hence there's a blockage somewhere. Any
suggestions leading me not to take off the power head? I don't have a manual for taking off the power
head but tips/hints is greatly appreciated :)


GH
 

OptsyEagle

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Re: 15hp low compression and then......small plastic pieces by theromostat

Sounds like an upper water grommet problem to me as well. Unfortuneately that one requires the powerhead to come off.

By the way, the copper tube will not come out with a good tug, on that motor. It is held into the exhaust housing by an over sized lip that requires it to come out the other way. You will see what I mean if you take off the powerhead.

If you do take off the powerhead, you might as well change the lower crank-case seal as well. They usually degrade after 35 years or so, are not expensive and easy to change once the powerhead is off.

Here's another link to a step by step procedure for removing a powerhead. I think it is from Bktheking in post #9.


http://forums.iboats.com/showthread.php?t=373779&highlight=transom+clamps
 

GreenHornet763

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Re: 15hp low compression and then......small plastic pieces by theromostat

Thanks alot guys! I also forgot that i remeasured the compression and it was at 88 and 84ish yesterday also? so it jumped roughly 14-18lbs of compression?
 

Jim311

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Re: 15hp low compression and then......small plastic pieces by theromostat

Did you replace the head gasket? That might have helped your compression numbers?


When I had a blockage from an impeller that blew up, I stuck a coat hang up the copper tube and spun it around a good bit, some rubber bits had accumulated up there into an almost total blockage. I also had blockages in the passages on the exhaust manifold. I had to take the plate off and replace the gaskets and clean out the blockage there. While the exhaust housing was off I blasted some water through the copper pipe with the garden hose connected. I bolted the exhaust plate back up, and remove the thermostat, and again forced water up and out of the thermostat housing. I am not familiar with the older series of motors you have, though, they probably have a different design, but if not, maybe these tips will help you.
 

Rick.

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Re: 15hp low compression and then......small plastic pieces by theromostat

Thanks alot guys! I also forgot that i remeasured the compression and it was at 88 and 84ish yesterday also? so it jumped roughly 14-18lbs of compression?
Well your making some progress. Why do you think your compression came up? Does your motor have a piped tell tale or are you just looking for water out the exhaust near the top of the back of the leg. I'm ashamed to bring this up this late but many times the lack of flow has been attributed to the water in the barrel not being high enough up the leg. Make sure the leg is as deep in the water as you can get it. With a hose connected to the water tube and the head removed try picking away at the water passages that feed the head. It may self clean once it starts passing water. You can also try back washing from those ports and see if the debris will come out. The idea of it running about two minutes and then quitting leads me to wonder if it's running out of fuel. Try pumping your bulb every minute to see if it runs longer. Rick.
 

jmendoza

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Re: 15hp low compression and then......small plastic pieces by theromostat

Be careful, don't put your leg in the barrel with the engine running unless you want to lose your toes and foot :).
As I have humorously pointed out before guys, if outboards could walk on water, they would have legs, but they can't and so they do not have legs! What they do have however is a mid section/exhaust housing, and a lower unit/gearcase. Some of our friendly Canuck neighbors insist on using non-standard jargon which only confuses the neophyte when he tries to translate the responses he gets to his questions. TIC (tongue in cheek) LOL.

Anyway, the 1974-76 models have an exhaust relief hole at the top of the mid section housing which will emit a spray of water only after the housing fills up with water, which takes some time, even with the engine in a barrel of water. The reason the housing fills with water is to silence the exhaust (for fishing/trolling) and to cool it for better flow and scavenging (from OMC service manual). Often the broken impeller blades will clog up the cooling system and copper water tube. My engine had poor water flow due to a partially collapsed upper water tube grommet.

As to your compression readings:
Be sure the throttle is wide open.
Do at least 6 pulls or more, or until the gauge readings do not increase.
 

Rick.

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3,740
Re: 15hp low compression and then......small plastic pieces by theromostat

Be careful, don't put your leg in the barrel with the engine running unless you want to lose your toes and foot :).
As I have humorously pointed out before guys, if outboards could walk on water, they would have legs, but they can't and so they do not have legs! What they do have however is a mid section/exhaust housing, and a lower unit/gearcase. Some of our friendly Canuck neighbors insist on using non-standard jargon which only confuses the neophyte when he tries to translate the responses he gets to his questions. TIC (tongue in cheek) LOL.

Anyway, the 1974-76 models have an exhaust relief hole at the top of the mid section housing which will emit a spray of water only after the housing fills up with water, which takes some time, even with the engine in a barrel of water. The reason the housing fills with water is to silence the exhaust (for fishing/trolling) and to cool it for better flow and scavenging (from OMC service manual). Often the broken impeller blades will clog up the cooling system and copper water tube. My engine had poor water flow due to a partially collapsed upper water tube grommet.

As to your compression readings:
Be sure the throttle is wide open.
Do at least 6 pulls or more, or until the gauge readings do not increase.

jmendoza:
I know your just pulling my leg... but point well taken and I'll try to get it right. Your warning made me laugh out loud (after I read it the second time). But, who says we're friendly? Rick.
 
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