70s Mercury 9.8 HP sputtering-one cylinder-rough running

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kevin99

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Joined
Jun 20, 2012
Messages
2
I got this a couple years ago. It ran great the first few times out. We got to Canada & it seemed to start missing. At first, it was random, then it got worse. I changed out the plugs, even though they appeared fine.

I noted that the pressurized gas tank was leaking a bit at the top rim where you put the gas in it. Little tiny pin holes that kinda foamed up with gas. I sealed that up & it appeared to fix that issue. I was thinking that possible WATER got into the tank OR that the leak was causing the gas to not get to the engine properly DUE to lack of pressure inside the tank.

EIther way, still have the issue. I'm changing out the gas now. Not sure if there is anther item that would make it run rough like this. It is odd, one second it runs great, then it just starts running on one cylinder & then back again. Like there is a lose wire. CHecked wires that I can see, and all appears solid.

Any help would be great. Thanks, Kevin
 
cr2k

cr2k

Captain
Joined
Mar 19, 2009
Messages
3,730
Re: 70s Mercury 9.8 HP sputtering-one cylinder-rough running

You only have one carb so it kinda narrows it the the electrical/ignition system. If you have points probably time for a new set. If you have a switchbox you either have stator problems or the switchbox is going bad.

With the engine running, and the cover off, gently wiggle the wires that go up under the flywheel and see if that makes anything change.

Make sure the coils are tight and grounded too.
 
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kevin99

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Joined
Jun 20, 2012
Messages
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Re: 70s Mercury 9.8 HP sputtering-one cylinder-rough running

I check that all out this weekend & report back. Hopefully I'll get it cured...thanks
 
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mercuk

Petty Officer 2nd Class
Joined
Feb 6, 2012
Messages
122
Re: 70s Mercury 9.8 HP sputtering-one cylinder-rough running

kevin99 said:
I got this a couple years ago. It ran great the first few times out. We got to Canada & it seemed to start missing. At first, it was random, then it got worse. I changed out the plugs, even though they appeared fine.

I noted that the pressurized gas tank was leaking a bit at the top rim where you put the gas in it. Little tiny pin holes that kinda foamed up with gas. I sealed that up & it appeared to fix that issue. I was thinking that possible WATER got into the tank OR that the leak was causing the gas to not get to the engine properly DUE to lack of pressure inside the tank.

EIther way, still have the issue. I'm changing out the gas now. Not sure if there is anther item that would make it run rough like this. It is odd, one second it runs great, then it just starts running on one cylinder & then back again. Like there is a lose wire. CHecked wires that I can see, and all appears solid.

Any help would be great. Thanks, Kevin
Click to expand...

Kevin,
Sounds to me as if you have a bad coil. I had exactly the same issue and it just turned out to be a failing coil. If its a THUNDERBOLT ignition (not earlier phelon system) then I wouldn't even worry about points as they rarely give any problems..
Thats were I would start anyways... good luck and let us know how ya get on...:D
 
O

oldman570

Lieutenant Commander
Joined
Mar 25, 2011
Messages
1,615
Re: 70s Mercury 9.8 HP sputtering-one cylinder-rough running

On the eaely 9.8 Mercs there are several things that could cause the trouble you describe. The kill button could be corroed and going bad, the coils could be breaking down, the wires to the points can fire accross where mounted in the stator plate, the stator itself could be goin bad, and a plug wire , if not the stranded type could be bad. The points on the older motors are maker points and supply greound ti the coils inorder for them to fire and do not burn nor pit as one would think. The setting of the points can be very tricky and are probbly not the problem, but is possable. Try running the motor at night with the band off and see if you can see any sparking wires or move the wires alittle while it is running and see if it acts up or quits acting up while doing so.
Oldman570
 
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