Johnson 9.9 loosing power

[GYPSY400]

Good day all,
Ok, so it's my turn to ask for help.
I have a 2000 9.9 Johnson model# J10RSSA
It will troll all day long without missing a beat, so I take it out and put it on the lake.. runs good for about 10 - 15 minutes, then looses RPM ( not sure how much, but a considerable amount) so my speed is about half of what it should be... the motor doesn't die right out, but looses power. And yes I prime the bulb when it happens.

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[Pappy]

Is it a sudden loss or a surge style loss of power?
Your half power comment leads me to believe it may be dropping a cylinder.
Can you tell if it is running on one or both at an idle? if you come off plane then get on plane again does the power return?

 

[GYPSY400]

Is it a sudden loss or a surge style loss of power?
Your half power comment leads me to believe it may be dropping a cylinder.
Can you tell if it is running on one or both at an idle? if you come off plane then get on plane again does the power return?

I would say it's a gradual loss, it doesn't surge but it's like your slowly throttling down. It doesn't feel like it's missing ( running on 1) at speed, but I'll have to get out and retest it once the ice leaves.
At idle it's very smooth and I'm pretty confident it's running on both.
If I throttle back and restart, most of the power comes back but it's not 100% like when it's cold. ( I'm guessing your thinking float level under that scenario?).

This motor has sat in my basement for 10 years due to low compression. I finally had time and money to get into it last summer, found out the head gasket was bad and replaced it. Checked out the carb and it was perfectly clean ( I was shocked) . Got it running in a test barrel and it trolled absolutely perfect for an hour. Took it out on the boat and it looses power, but doesn't die right out. Then I ran out of summer before getting to diagnose it further.

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[poorthang]

what was the cause of the original head gasket blowing? :idea:

 

[GYPSY400]

what was the cause of the original head gasket blowing? :idea:

It's a poor design.. not much space between the cylinders and very common on this model.

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[poorthang]

so, if that is true, do you reckon history repeats?

 

[Pappy]

what was the cause of the original head gasket blowing? :idea:

It's a poor design.. not much space between the cylinders and very common on this model.

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Ahhh, so you are an engineer now?
That particular design has been in that engine since 1974. There are probably hundreds of thousands of those engines out there and running or worn out with that gasket still in them.
In short, you did not discover why the gasket failed. Period.
Pull the powerhead and you will see that the grommet on the water tube has melted and swollen shut. Probably from numerous overheats.
This is restricting your water flow to the powerhead. This has been discussed in here a few times in the past. Photos are available as well.

 

[poorthang]

pappy condensed story......its getting hot and dragging the pistons. your local info is not reliable. pappy hit it in the head.
peace

 

[Pappy]

Take a look at this thread for example. Should show you what is going on with your engine.
https://forum.tinboats.net/viewtopic.php?p=432188#p432188

 

[GYPSY400]

what was the cause of the original head gasket blowing? :idea:

It's a poor design.. not much space between the cylinders and very common on this model.

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Ahhh, so you are an engineer now?
That particular design has been in that engine since 1974. There are probably hundreds of thousands of those engines out there and running or worn out with that gasket still in them.
In short, you did not discover why the gasket failed. Period.
Pull the powerhead and you will see that the grommet on the water tube has melted and swollen shut. Probably from numerous overheats.
This is restricting your water flow to the powerhead. This has been discussed in here a few times in the past. Photos are available as well.

Ok I'll take a look at that grommet you speak of. I never thought overheating would be an issue as I replaced the water pump at the same time as the head gasket and pee stream was strong.

I realize that the 9.9 has a very good reliability record, hence why I bought it Years ago. I thought on the newer style engines that they moved the cylinders closer together thus bringing up head gasket issues.

When I bought this motor, it ran good for the first year, then became hard to start in the second year and eventually failed... I later find out it was a lodge motor and probably has a million hours on it..



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[GYPSY400]

Take a look at this thread for example. Should show you what is going on with your engine.
https://forum.tinboats.net/viewtopic.php?p=432188#p432188

Thank you, that thread has alot of useful information. I never would have thought to look there for an overheating issue as the telltale was strong. But it does make sense on how the engine is performing, hopefully the cylinders are still in good shape after this overheating event ( I have about 2hrs of run time on it at higher rpm)
Funny thing is, I did a similar repair on a 4hp mercury last summer - motor would pee good at idle , but blow steam at higher rpm.

Just to be clear, the grommets you speak of are items 17 and 19 in the diagram?

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[Pappy]

Yep.
While there have new fuel line ready. Will be easy to replace while the powerhead is off. Same with the overboard indicator hose. You will need a powerhead gasket for this job as well. If the engine needs carb work now is also the time to do it. Easy while the powerhead is off.
Since the engine has been overheated aagain it may be wise to check the head gasket and replace it as well as check and clean the thermostat. Again, easy to do while thee powerhead is off. Check for straightness on the cylinder head. Take a look at your cylinder walls as well to see if they are scuffed.

 

[GYPSY400]

Yep.
While there have new fuel line ready. Will be easy to replace while the powerhead is off. Same with the overboard indicator hose. You will need a powerhead gasket for this job as well. If the engine needs carb work now is also the time to do it. Easy while the powerhead is off.
Since the engine has been overheated aagain it may be wise to check the head gasket and replace it as well as check and clean the thermostat. Again, easy to do while thee powerhead is off. Check for straightness on the cylinder head. Take a look at your cylinder walls as well to see if they are scuffed.

Yes , all good points to check while I'm in there.. I'm going to order parts tomorrow and I'll update once I get into it for anyone having the same problem in the future.

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[wmk0002]

Out of curiosity what were your new baseline compression results after the new head gasket and have you checked them since?

 

[Kismet]

Once again, I am in awe of the expert, free diagnostic analysis that Pappy gives so generously.

We're pretty lucky here, folks.


=D> =D> =D> =D>

 

[onthewater102]

I bought one of these used and I'm due to replace the impeller...guess I'll be checking into this proactively now too!

 

[GYPSY400]

Out of curiosity what were your new baseline compression results after the new head gasket and have you checked them since?

I don't recall the exact number after the head-gasket replacement.. I can't even remember if I checked it.. but I came to the conclusion it was a head gasket because the compression number changed if the other spark plug was in or out.

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[GYPSY400]

Once again, I am in awe of the expert, free diagnostic analysis that Pappy gives so generously.

We're pretty lucky here, folks.


=D> =D> =D> =D>

Yes your absolutely right!! I was thinking it was a partially failing CDI or coil. I was very confident the fuel system was up to snuff so I ruled that out. And when I did the head gasket I did the water pump and thermostat to rule out the cooling system. I never would have thought about the grommets at the exhaust tube as it was peeing good at all rpm.

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[GYPSY400]

Ok, finally got a chance to tear into this 9.9. Removed the power head from the chassis and removed the exhaust tube. Found that the grommets were not swollen shut - I dont think this design would allow a water restriction as the grommet is outside the tube and flush with the end.

I did notice an excessive amount of carbon Schmoo on the exhaust pipe - inside and outside. Also, after cleaning I found a pinhole in the longer tube, not sure if it's there on purpose.

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[Pappy]

The pin hole is there on purpose.