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Yamaha 90hp One VERY clean Spark Plug?

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  • Yamaha 90hp One VERY clean Spark Plug?

    Hi Guys, I have a Yamaha 90hp engine. It runs well, has very good compression. (120, 118, 119), BUT I noticed that on the bottom cylinder, the spark plug is always amazingly clean. Like Brand new. No oil, gas, carbon, like new. The other plugs look like they do from a motor, slightly discolored on the electrode, a hint of oil, I think you know what I mean. I am wondering why the #3 spark plug (lowest of inline 3 cylinder) is like this. I once read that it could possible be a air leak or maybe water getting in somehow. Tell me what you think guys. FWIW, I don't think the head gasket has ever been changed and this is 12 years old.Thanks,John

  • #2
    Re: Yamaha 90hp One VERY clean Spark Plug?

    John, I am not well versed in yammi's, but in a 2 stroke outboard a clean plug means one of 2 things...water getting in the combustion chamber or a non functional cylinder...if the motor peforms as normal you can assume a small amount of water in the cylinder when running...usually from a gasket....

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    • #3
      Re: Yamaha 90hp One VERY clean Spark Plug?

      Sounds like you are getting some water, John.Most likely (and cheapest) cause is a leaky head gasket, but your compression doesn't support that. Maybe someone with more Yammy experience has a better suggestion, but I would pull the head and look for what else it might be. Yammys don't break very often, so we don't get to see very many troubles.Good luck, and let us know what you find.Red sky at night. . .JB
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      • #4
        Re: Yamaha 90hp One VERY clean Spark Plug?

        Ditto ODD1 And JB. You've got an H2O leak. I would suspect a head gasket.If you find that's what it is, you can stop that from happening again by retorquing the head after a complete engine warm up.While you've got the head off, take a ggod look at that cylinder to make sure it's not pitted.

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        • #5
          Re: Yamaha 90hp One VERY clean Spark Plug?

          Guys, Thanks. I guess I will try a new head gasket. With this one being over 12 years old, I suppose it's time to change it anyway. Do head gasket's have a normal change interval? Now..if I can find that darn Yammie manual to tell me which bolt to screw in first.....Thanks again.John

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          • #6
            Re: Yamaha 90hp One VERY clean Spark Plug?

            Any of you fine gents have the torque and sequence diagram for this inline 3 cylinder Yammie (1989)? (I'm refereing to the head bolts)Thanks a zillion.John

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            • #7
              Re: Yamaha 90hp One VERY clean Spark Plug?

              John, I am about to purchase a motor with similarsymptoms. Just wondered what the result of your experience was?Cheers,Roly~

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              • #8
                Re: Yamaha 90hp One VERY clean Spark Plug?

                John,Sorry to hear you have not fixed the motor yet. If you have a fax, put the number up on the board or email me and I will fax the torque spec/sequence to you.Good Luck

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                • #9
                  Re: Yamaha 90hp One VERY clean Spark Plug?

                  John I was wondering if replacing the head gasket solved your problem. I've got exactly the same symptoms on my 40hp yammie, clean #3 spark plug but good compression.

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                  • #10
                    Re: Yamaha 90hp One VERY clean Spark Plug?

                    Hey guys this could mean you have a hot running engine as well the water circulates from top to bottom in these in line motors check your T-stat it may be late opening. with tese old motors the salt or even mineral build up can insulate the thermastat also if there was enough water getting in to make the plug this clean you notice a low speed miss and the plug would have a white reidue left on it I think you should check to see if it is running correctly.also is the motor oil injected? how many hours on this motor? when the boat is on the trailer is there any oil residue running out of the exhaust? it there is what colour is it?your problem does not point directly to a head gasket but may be somthing even simpler the yamy hand book sugest that every 4 yrs the head gaskets should be changed and also the anodes in the headse should eather be changed or removed all to geather they can break and cause a blockage in the power head. good luck any questions Email me at cartermarine@yahoo.com Thanks Trevor

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                    • #11
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                      Re: Yamaha 90hp One VERY clean Spark Plug?

                      Just a long delayed follow up on this motor.I removed the head gasket and found it to be nearly perfect, and all the cylinder heads and walls looked perfect and the same.The cylinder with the "clean" spark plug looked no different. I was worried that water injestion was cleaning the cylinder walls too and de-lubing it. This was not the case.The motor ran and performed flawlessly so I left it alone after only installing the new head gasket.I since sold it and it is running great.John from Madison, CT

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