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9.9 1974 Johnson Heavy Black On Plug

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patiolatern

Regular Contributor
10R74S 9.9. ……could be a long one
Rebuilt carb 6 weeks ago with OEM kit started well and ran in barrel. Adjusted lean/rich mixture at the time and no issues in barrel. Have not run it on water until this weekend.

Went out trolling and started the 9.9 rather then run my 70hp. The 9.9 seemed to push a lot of air bubbles and it seemed slow or having a harder time pushing the boat-16 ft aluminum bow rider.
It shut down after 30 minutes and could not get it started. Took the plugs out and the bottom plug was completely black with spit/carbon. The top plug seemed fine.

Today I checked a few things:
Compression is 120 on both cylinders
Spark was strong on both cylinders
Fuel pump was replaced over a year ago so I took it apart and it appears fine.
Checked spark plugs with ohm reading and seems fine

Switched fuel tanks to rule that as an issues for testing today

It was hard to start and keep running but I was able to and put it into gear. Seemed to run below normal levels at WOT. Run for a bit and then would stall out. Checked plugs and I think the top one had all black on it but I sort of lost track on what plug was where so not 100% sure. Cleaned spark plugs and ram it a few more times still hard to start and still stalls out. Tried another set of plugs and same issues.

Had a cheap tachometer on the bottom cylinder but it would not pick up a reading. Switched it to the top cylinder and was getting a reading of around 3500rpm. I also took then plug wire off the bottom cylinder while running and it didn’t run any worse. I did not get a chance to do this to the top cylinder.

Put a temperature sensor to the engine while running and it seems the top cylinder was getting warmer but the bottom cylinder lagged behind by a significant amount.

Not sure what this points to and what I can further diagnose to get to the issue and get this running right.

Is it my lean rich screw? Such a pain to start and keep running it’s hard to try different things. I would like to confirm all the problems are on the bottom cylinder but I can’t until it runs a little longer?
Would a bad coil cause this(even with strong spark)?

Any ideas or help is appreciated!!
Oh yeah….thanks for reading this long ass text.

Edit: My barrel water had a lot of black film in it.
 
I'm just learning about these 9.9s, so if my ignorance shows, forgive me.
While reading your issues my first thought was to swap the position of the ignition parts particular to each cylinder and see if the issue follows from cylinder to cylinder. By that I mean to swap coils (assuming you have them) and even the wire to see if the problem follows. Then I got curious and looked up your motor: surprised me to see points and condensers. If that's the case, maybe swap the position of the spark plug wires first, then the condensers and see what happens?
You mentioned the idle mixture screw, but seems to me that would affect both cylinders equally.
 
First thing to note is that your 9.9Hp is of a model series that will not serve you very well for trolling. The 1974-1976 9.9Hp/15Hp will foul spark plugs very quickly if you run them for extended times at low speeds. If you do, you should use NGK B7HS spark plugs and always run it WOT for a while when you come back into the dock/boat launch, to hopefully burn off some of that carbon build up. In 1977 OMC changed their entire ignition system, most likely to address this very issue.

Anyway, you need to do a cylinder drop test. Start up the motor, warm it up and bring it to a fast idle and pull one spark plug boot at a time. If it dies when a spark plug boot is removed it tells you that the other cylinder was not firing or was very weak. If you are running on one cylinder my best guess is your spark plug is fouled but verify all that first and we can go from there.
 
racerone said:
I do not believe spark is strong on both cylinders !----Does spark jump a gap of 5/16" ?
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I know. Very good spark on both. Used a drill to run it and seemed pretty strong….easy 1/2” if not more but I will run through everything again just to make sure.
 
Post #3 says it all. Those things were notorious for fouling plugs. Some wouldn't run one weekend. Try NEW plugs and see what happens.

Yeah, I know, somebody will come back and tell how well theirs has always run.
 
patiolatern said:
I know. Very good spark on both. Used a drill to run it and seemed pretty strong….easy 1/2” if not more but I will run through everything again just to make sure.
Click to expand...
Pic one is bottom cylinder and pic 2 is top one. Let me know what you think. Tried to upload a short video but wouldn’t locate it.
 

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optsyeagle said:
First thing to note is that your 9.9Hp is of a model series that will not serve you very well for trolling. The 1974-1976 9.9Hp/15Hp will foul spark plugs very quickly if you run them for extended times at low speeds. If you do, you should use NGK B7HS spark plugs and always run it WOT for a while when you come back into the dock/boat launch, to hopefully burn off some of that carbon build up. In 1977 OMC changed their entire ignition system, most likely to address this very issue.

Anyway, you need to do a cylinder drop test. Start up the motor, warm it up and bring it to a fast idle and pull one spark plug boot at a time. If it dies when a spark plug boot is removed it tells you that the other cylinder was not firing or was very weak. If you are running on one cylinder my best guess is your spark plug is fouled but verify all that first and we can go from there.
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Ok so did a bunch of testing and so far this what I have:
Put in new spark plugs and it fired up right away and seemed to have some power that was lacking. It did idle to high around 1800rpm in neutral switched to forward and rpm went to about 1100.
I had consistent temp on each spark plug of around 120 f or so.
Cylinder temp from side of motor showed top at around 145-150 f and bottom around 125-130 f
The back of motor (head gasket?) showed about 120-135 and about the same across the face of it. These temps aren’t perfect but they are consistent with top being hotter then bottom by 20 degrees f
The water coming from top exhaust seemed hotter then you would like more hot then warm you could leave your hand in it but….

The motor would sneeze on occasion at lower speed or in neutral.

Once it stalled it was very difficult to get started again
Took plugs out and the top plug looks to be fouling….slightly but different from the very clean bottom plug. Switched the plugs and they both now look the same-the top cylinder fouled the other plug a little.

When I had the motor running I pulled the top plug wire off and the motor continued to run. After putting the top wire back on I pulled the bottom wire off and it continued to run

Next steps? Swap coils and see if the fouling follows? And???
 
patiolatern said:
NGK B7HS
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Swapped over the coils and it seems about the same as before. Where I took temps the top cyl is about 145 and the bottom is now about the same. Probably caused by the angle I scan it. It is hot to the touch on both of the flat areas in the pic where I am getting 145. Can’t hold your hand to it for very long. I will run for a few and see what the spark plugs look like.
 
patiolatern said:
Swapped over the coils and it seems about the same as before. Where I took temps the top cyl is about 145 and the bottom is now about the same. Probably caused by the angle I scan it. It is hot to the touch on both of the flat areas in the pic where I am getting 145. Can’t hold your hand to it for very long. I will run for a few and see what the spark plugs look like.
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Pic
 
patiolatern said:
Pic
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patiolatern said:
Swapped over the coils and it seems about the same as before. Where I took temps the top cyl is about 145 and the bottom is now about the same. Probably caused by the angle I scan it. It is hot to the touch on both of the flat areas in the pic where I am getting 145. Can’t hold your hand to it for very long. I will run for a few and see what the spark plugs look like.
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Did some more reading on this motor and it seems the 145 f is the correct temperature and the water temp out the exhaust port seems to be about right too. I will leave the coils in their current position since there was no noticeable change. I will take it out for a little test drive tomorrow and see how it goes. Maybe I am being over sensitive to any spark plug discolouring but we should find out after I get back from a ride.
Thanks!
 
I wouldn't worry too much about the black on the plugs. As I said that motor does not burn its fuel the best so black spark plugs are going to happen. I think swapping out the plugs will probably solve the running issues but you have the motor and not me so I can't be sure. If it does, then just refrain from trolling a lot or be prepared to replace spark plugs very frequently.

If the motor does run well, it would not hurt to consider performing a decarb on the motor. If carbon was getting on the spark plugs enough to foul them you can be sure it was also getting on the cylinders and piston rings as well. A seafoam treatment or powertune would be my next suggestion.
 
optsyeagle said:
I wouldn't worry too much about the black on the plugs. As I said that motor does not burn its fuel the best so black spark plugs are going to happen. I think swapping out the plugs will probably solve the running issues but you have the motor and not me so I can't be sure. If it does, then just refrain from trolling a lot or be prepared to replace spark plugs very frequently.

If the motor does run well, it would not hurt to consider performing a decarb on the motor. If carbon was getting on the spark plugs enough to foul them you can be sure it was also getting on the cylinders and piston rings as well. A seafoam treatment or powertune would be my next suggestion.
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Thanks for the feedback. Hopefully everything runs normal tomorrow. I have never had spark plugs go bad or fail before so I am surprised.
Is there a way to test if they appear to fail in the future? Is it as simple as measuring ohm accurately to spec of plug? Any other ways or is it just a thing to watch for and a simple and cheap fix?
I will do the sea foam treatment once I pick some up. Thanks for the advice!
 
Usually only one plug fails first so you can usually get back to the dock/launch on the other cylinder. Some people keep an extra set of spark plugs in the boat but unless you are really far away I would just go home slowly. Since you never know for sure what is causing your loss of power it does not hurt to do a cylinder drop test to confirm the issue, when you get back. That model year is prone to fouling spark plugs. I owned a 1975 model and I know the problem well. I just used it for my main motor, usually run close to WOT or mid range and if I wanted to troll I had an electric trolling motor for that. You could also go to NGK B6HS but don't go any hotter on the plugs then that.

Anyway, we are ahead of ourselves here so test the motor and let us know how it all works out for you.
 
That motor it is fairly easy to see how your T-stat is working, or not working, or if it is not installed at all. That is because most of the water that comes out the exhaust relief hole needs to go through the T-stat, which is not the case in a lot of other models. So when you first fire up the motor you will notice that it is almost impossible to see the water coming out the back of the exhaust relief hole. If you put your hand back there you can feel a slight mist. After about 20 to 30 seconds you will start to see a nice spray of water as the T-stat opens. When out in the water running faster in gear you will see that spray turn to a stream that moves back and forth a little as the T-stat opens and closes.

If you see a spray of water immediately after firing up the motor I would guess there is no T-stat or it has failed open.
 
optsyeagle said:
Usually only one plug fails first so you can usually get back to the dock/launch on the other cylinder. Some people keep an extra set of spark plugs in the boat but unless you are really far away I would just go home slowly. Since you never know for sure what is causing your loss of power it does not hurt to do a cylinder drop test to confirm the issue, when you get back. That model year is prone to fouling spark plugs. I owned a 1975 model and I know the problem well. I just used it for my main motor, usually run close to WOT or mid range and if I wanted to troll I had an electric trolling motor for that. You could also go to NGK B6HS but don't go any hotter on the plugs then that.

Anyway, we are ahead of ourselves here so test the motor and let us know how it all works out for you.
Click to expand...
Well it started off great! Motor started right away and warmed up nicely. Put it into gear and it had pep so very good. Then after about 5 min at 3/4 throttle it shut down. Started it again and was smooth in idle then went into gear at low throttle and it would run fun doing about 2 mph and around 1800rpm. Gave it some more throttle and it did not like that at about 2000 rpm and would stall out. Rinse and repeat. It then would not start so took plugs out to check and they were black carbon covered. Cleaned them up and still did not want to fire up.
Pulled it from the water and take it back to the barrel to problem solve.
Any ideas?
 
optsyeagle said:
That motor it is fairly easy to see how your T-stat is working, or not working, or if it is not installed at all. That is because most of the water that comes out the exhaust relief hole needs to go through the T-stat, which is not the case in a lot of other models. So when you first fire up the motor you will notice that it is almost impossible to see the water coming out the back of the exhaust relief hole. If you put your hand back there you can feel a slight mist. After about 20 to 30 seconds you will start to see a nice spray of water as the T-stat opens. When out in the water running faster in gear you will see that spray turn to a stream that moves back and forth a little as the T-stat opens and closes.

If you see a spray of water immediately after firing up the motor I would guess there is no T-stat or it has failed open.
Click to expand...
Yep that’s exactly what it’s doing. No water at first but after some idle there is some water not a lot but lots when it’s in gear on the open water
 
OK. So you have a T-stat and it appears to be working fine.

This would be a good time to do a cylinder drop test to determine if both cylinders are firing. I explained the procedure up thread.

Also, check your link and sync. As you advance the throttle you will notice that a throttle cam from under the flywheel moves along and pushed on the roller on your carburetor. The spark advance system of that motor is set properly when the indented arrow on that throttle cam is in the center of the roller when it JUST touches the roller. I am wonder if the spark timing is firing in the wrong place as you advance the throttle to higher RPMs. This would also cause issue for fuel burn at lower RPMs effecting your spark plugs, at least the color of them after use anyway.
 
optsyeagle said:
OK. So you have a T-stat and it appears to be working fine.

This would be a good time to do a cylinder drop test to determine if both cylinders are firing. I explained the procedure up thread.

Also, check your link and sync. As you advance the throttle you will notice that a throttle cam from under the flywheel moves along and pushed on the roller on your carburetor. The spark advance system of that motor is set properly when the indented arrow on that throttle cam is in the center of the roller when it JUST touches the roller. I am wonder if the spark timing is firing in the wrong place as you advance the throttle to higher RPMs. This would also cause issue for fuel burn at lower RPMs effecting your spark plugs, at least the color of them after use anyway.
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Will get to this first thing in the am!! Thanks again for all your help!
My pull rope was in serious need of changing so I was working on that for the past hour or so. They run that rope so that you have to disassemble the whole thing to get the rope out of the recoiler. I have to take the recoiler off to tighten the rope up a little tomorrow as well. Since I have the air intake and recoiler off is there any benefit to pulling the carb off and getting at the reeds to inspect? Means taking the cover off as well and probably a new gasket I don’t have yet. Maybe leave this for next stage after I do the checks above?

Fyi when running at low idle the motor would sneeze. If idle went to low and it sneezed then it shut down.
 
I would stay away from the reeds for now. It is a bigger job and reeds rarely fail.

Do the cylinder drop test and if both cylinders are firing then perhaps look at setting your rich/lean adjustment again. Perhaps you are too lean for low idle. Also, look into the link and sync status as well.
 
flyingscott said:
Check for spark when that is happening. Could have a bad condenser or coil. What is your compression?
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Hi flyingscott! We have 120 psi on both cylinders and we have a strong visual spark on both cylinders. We did a cylinder drop test and motor continued to run each time. Tomorrow I will do it again as well as what optsyeagle has suggested.
I will post results.
Thanks !
 
flyingscott said:
When was the last time the carb was cleaned and a good kit installed? Choke flapper returning to full open?
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Carb rebuild was done about a month ago with OEM kit. Everything looked good. I can double check choke in the am to make sure everything looks good as well as the other ideas like the lean/rich mix and throttle position and drop test etc.
 
optsyeagle said:
I would stay away from the reeds for now. It is a bigger job and reeds rarely fail.

Do the cylinder drop test and if both cylinders are firing then perhaps look at setting your rich/lean adjustment again. Perhaps you are too lean for low idle. Also, look into the link and sync status as well.
Click to expand...
Ok here we go:
The roller for the link and sync seemed to be in the correct position maybe a hair late in moving the roller.
Since I had the recoiler off I took the carb off and gave it a cleaning. The idle jet was at 1 1/4 turns out. Nothing found in carb. During cleaning.
Assembled it back to the motor and butterfly plates seemed to work properly as best I can tell. The internal plate(not choke) is working since I was able to put a zip tie in to feel it activate.
Put the roller for link and sync was put into the exact position required.
Assembled the recoiler and put the flywheel back on. Points were checked and were correct at .020”.
To start the engine I had to lean the idle mix by a 1/4 turn. Engine started and let it warm up watching thermostat open etc
After warming up the engine had pep and seemed to run ok. Was able to adjust the lean/rich to get all or most sneezes out of it at idle.
Ran engine in gear at medium speed and everything seemed to ok. Temp scans showed 145 f or so and after a while this climbed to 155 -160 then when I slowed down the temp dropped back to 145 or so. Water jacket on back end stayed around 110-120f.
Drop test was performed and engine continued to run after each spark plug was disconnected.

Ran for a fair amount of time and had idle down to around 750-800rpm in idle with limited sneezes so this seemed ok
Then I put it in gear and things seemed ok at lower rpm but then I started to accelerate and there was a lag as slowly the rpm’s came up to speed (with some excess smoke) and the rpm really picked up then it slowed right down and dies.
Plugs were blackened so cleaned them and it was hard starting again and after starting it did the same situation occurred.
Not sure how to describe this but it seems fine until warmed up and higher speeds are demanded then it says no thanks! Could it be dropping a cylinder at this point?
Once again I am open to any and all feedback and advice!
Thanks !
 
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