Yamaha 90hp One VERY clean Spark Plug?

John from Madison CT

Petty Officer 2nd Class
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Oct 14, 2001
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176
Hi Guys,<br /><br /> 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.<br /><br /> 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.<br /><br /> Tell me what you think guys. FWIW, I don't think the head gasket has ever been changed and this is 12 years old.<br /><br />Thanks,<br /><br />John
 

ODDD1

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Jan 23, 2001
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1,054
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....
 

JB

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Mar 25, 2001
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45,907
Re: Yamaha 90hp One VERY clean Spark Plug?

Sounds like you are getting some water, John.<br />Most likely (and cheapest) cause is a leaky head gasket, but your compression doesn't support that. :confused: <br /><br />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.<br /><br />Good luck, and let us know what you find.<br /><br />Red sky at night. . .<br />JB :)
 
D

DJ

Guest
Re: Yamaha 90hp One VERY clean Spark Plug?

Ditto ODD1 And JB. You've got an H2O leak. I would suspect a head gasket.<br /><br />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.<br /><br />While you've got the head off, take a ggod look at that cylinder to make sure it's not pitted.
 

John from Madison CT

Petty Officer 2nd Class
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Oct 14, 2001
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176
Re: Yamaha 90hp One VERY clean Spark Plug?

Guys,<br /><br /> 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?<br /><br /> Now..if I can find that darn Yammie manual to tell me which bolt to screw in first.....<br /><br />Thanks again.<br /><br />John
 

John from Madison CT

Petty Officer 2nd Class
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Oct 14, 2001
Messages
176
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)<br /><br />Thanks a zillion.<br /><br />John
 

Roly

Recruit
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Jul 13, 2002
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4
Re: Yamaha 90hp One VERY clean Spark Plug?

John, I am about to purchase a motor with similar<br />symptoms. Just wondered what the result of your experience was?<br />Cheers,<br />Roly~
 

araiha

Petty Officer 2nd Class
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Sep 8, 2001
Messages
121
Re: Yamaha 90hp One VERY clean Spark Plug?

John,<br /><br />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.<br /><br />Good Luck
 

rastoga

Cadet
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Oct 15, 2001
Messages
23
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.
 

Trevor

Petty Officer 3rd Class
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Aug 12, 2002
Messages
92
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 <br />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.<br />also is the motor oil injected? <br />how many hours on this motor? <br />when the boat is on the trailer is there any oil residue running out of the exhaust? it there is what colour is it?<br />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 <br />Thanks Trevor ;)
 

John from Madison CT

Petty Officer 2nd Class
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Oct 14, 2001
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176
Re: Yamaha 90hp One VERY clean Spark Plug?

Just a long delayed follow up on this motor.<br /><br />I removed the head gasket and found it to be nearly perfect, and all the cylinder heads and walls looked perfect and the same.<br /><br />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.<br /><br />The motor ran and performed flawlessly so I left it alone after only installing the new head gasket.<br /><br />I since sold it and it is running great.<br /><br />John from Madison, CT
 
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