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Compression Test XJ650

Discussion in 'XJ DIY How-To Videos' started by taptapindustries, May 19, 2017.

  1. taptapindustries

    taptapindustries Member

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  2. taptapindustries

    taptapindustries Member

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    compression test on my XJ650. hope this helps people out there and or at least entertains. :)
     
  3. XJ550H

    XJ550H Well-Known Member Premium Member

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    you would want to disconnect the TCI before starting your compression test.
     
  4. a100man

    a100man Well-Known Member

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    Why is that?
     
  5. Rooster53

    Rooster53 Well-Known Member Premium Member

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    To protect the coils. Yamaha suggests grounding the spark plugs, but it is easier in most cases just to disconnect the TCI. If the spark plugs are not grounded or the TCI disconnected the voltage in the secondary of the ignition coil that normally builds to a predetermined level set by the spark plug gap will build to a higher level. As the voltage in the secondary builds above the design, it will arc across the wire enamel insulation of the secondary and find its way to ground resulting in damage to the coil.
     
  6. a100man

    a100man Well-Known Member

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    Makes sense - thanks.
     
  7. Wintersdark

    Wintersdark Well-Known Member

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    The entire electrical system is off - it's just an engine on my bench, and one in an otherwise stripped frame. So, no problem there!
     
  8. Wintersdark

    Wintersdark Well-Known Member

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    Thanks, good sir, that was exactly what i needed to see :)
     
  9. Wintersdark

    Wintersdark Well-Known Member

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    Quick question: the motors I need to test have neither carbs nor exhaust attached. I'd assume this shouldn't be a problem (you want wide open to test, hence wide open throttle, no?) But as neither intake nor exhaust are connected, just wanted to confirm that won't skew results.
     
  10. XJ550H

    XJ550H Well-Known Member Premium Member

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    test it as it is , it may give you better numbers then when assembled but it will still give you some insight as to the condition of the motor
     
    Wintersdark likes this.

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