Spark Plug Appearance

Discussion in 'Touring Models' started by tourglide, Apr 29, 2013.

  1. tourglide

    tourglide Member

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    I just pulled the plugs on my carburettor fueled 80 cu in Evo. the rear cyl plug was sooty with a dark insulator the front plug was clean with a white insulator. Its not possible for the two cylinders to differ in mixture is it?
     
  2. 89stroker

    89stroker Active Member

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    I bought a 99 fxst with an 80 inch evo last year, found the plugs same as yours. I did some work over the winter with completely going through the carb, rebuilding it and rejetting it. I found some issues in the carb and the bike did run super rich at idle and in the midrange prior to this work. Have not had the chance to ride it this spring yet due to the extra long winter (got another foot of snow over night!!)
     
  3. HDDon

    HDDon Experienced Member Contributor Retired Moderators

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    Your EVO has the same manifold serving both cyl. How many miles on the motor? This could be as simple as the sparkplug wires not being seated in the coil or just bad wires.
     
  4. Breeze3at

    Breeze3at Well-Known Member

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    The rear cyl. on all Harleys runs a bit richer due to the 45 degree cylinders and single crankpin. The 45 degree causes firing to happen 315 and 405 crankshaft degrees apart (the reason Harleys idle so nice). That causes uneven cylinder fill due to the "ram" effect in the manifold being different. It's less of an effect on F.I., but still there. There's more to it than my simple explanation, and don't discount that there may be one of the problems as posted previously.
     
    Last edited: Apr 30, 2013
  5. Jack Klarich

    Jack Klarich Expert Member

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    Just for grins have you run a compression test? What jetting do you have in your carb?
     
  6. dbmg

    dbmg Experienced Member

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    On my 2008 EG I just replaced the plugs at the 25,000 mile service and every 5,000 mile previous intervals and plugs were the same way. Bike has always run like a champ. So my opinion is no harm or foul............
     
  7. tourglide

    tourglide Member

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    Thanks for all your suggestions. I think I am worrying too much as I had a completely rebuilt engine installed over winter. It's still cool so short trips and excessive idling to warm it up have not helped the spark plug appearance. Once we reach normal riding temperatures my experts have promised to check my carb settings.
     
  8. tourglide

    tourglide Member

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    As a follow up to my original post what is the correct procedure to "read" your spark plugs. I have been advised different ways including shutting the engine off at highway speed and coast to a stop. Thanks in advance.
     
  9. Jack Klarich

    Jack Klarich Expert Member

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    NGK Spark Plugs USA Here is a good link to read plugs, make a couple of hard runs , keep in mind with modern fuels it will be harder to read them, with a magnifier and a light you can view the insulator and also see the firing rings also called heat rings You will need to make your run at wide open throttle shifting thru all gears and shutting the bike down immediately after your test run to get an accurate reading
     
    Last edited: May 1, 2013
  10. Bodeen

    Bodeen Well-Known Member Staff Member Moderator Contributor

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    My neighbors love when I do this! :newsmile042: