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3 out 4 spark plugs fire up

Discussion in 'XJ Technical Chat' started by Grant Murray, Jul 21, 2020.

  1. Grant Murray

    Grant Murray New Member

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    Hi I'm new to the forum so hopefully putting this thread in the right section. I have a 1981 XJ650 4KO model and am having an issue with one spark plug not sparking. I have replaced the coils plugs leads and plug caps with new ones and also fitted a secondhand working CDI unit, however the same fault still persists. It doesn't matter which coils you swap round the exact same fault just reoccurs. Has anyone had any similar issues. I have owned the bike for about thirty years and decided to get it going again after it sat for 10 years in my garage.
     
  2. rocs82650

    rocs82650 Well-Known Member

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    Welcome aboard. Did you clean the carburetors?

    Gary H.
     
  3. Grant Murray

    Grant Murray New Member

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    Hi Yes all been cleaned and set up the issue is definately electrical.
     
  4. joejr2

    joejr2 Active Member Premium Member

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    #1 and #4 fire at the same time as do #2 and #3. If one plug fires and it's partner does not,
    it's not the coil. There is either a poor connection between the plug and it's coil at the boot,
    the plug is defective, or that cylinder is not getting enough gas. Clean the carbs again.
     
  5. Grant Murray

    Grant Murray New Member

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    after a lot of tracing wires etc it appears the new coil I bought is faulty. Company sending a new one out today. Thanks for eberyones help I will keep you posted of progress.
     
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  6. rocs82650

    rocs82650 Well-Known Member

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    Did a Ohm test reveal this?

    Gary H.
     
  7. SQLGuy

    SQLGuy Well-Known Member

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    One plug not firing (with the plug resting against the block) pretty much has to be a coil problem... because the two plugs fire in series. With series firing, if one plug is not firing but the other is, that means that the current is completing its circuit somewhere in the coil itself.
     
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  8. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

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    Or one plug is bad.
     
  9. rocs82650

    rocs82650 Well-Known Member

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    Exactly.

    Gary H.
     
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  10. Grant Murray

    Grant Murray New Member

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    Fault cured with new coil.
     
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  11. SpearChucker

    SpearChucker Active Member

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    So let's make sure I have this straight:
    - 1&4 are on the same coil
    - 2&3 are on the same coil
    - if 2 doesn't fire, 3 wouldn't either if it was a bad coil
    - if 3 fires, check HT wire, cap and plug on 2
     
    Last edited: Aug 19, 2020

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