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Xj750 mid max - 1983 - compression question

Discussion in 'XJ Technical Chat' started by Mkehughes678, Aug 13, 2019.

  1. Mkehughes678

    Mkehughes678 New Member

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    Hey Guys, Curious if anyone knows basic rough specs for cold engine compression on my 83 xj750 midnight maxim

    I'm about 400 feet above sea level if that makes a difference.

    With Spark plugs in and capped, my readings from left to right ( 1-2-3-4) were:
    131-105-110-107

    With all spark plugs removed, my readings from left to right ( 1-2-3-4) were:
    110-120-123-120

    Engine was cold and had a bit of help from a trickle charger set to jump. I know the battery is fine. However I did notice my 3 and 4 cylinders are not firing either. new plugs had a faint fuel smell but the prong on the plug was brand new on and clean on both 3 and 4, where as 1 and 2 smelt much more like fuel and were darker from firing.

    Looking at buying new coils, any advice?

    cheers!
     
  2. Polock

    Polock Well-Known Member

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    could be the plug wires or plug caps or even the plugs but it's probably the carbs
    don't buy coils yet
     
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  3. Jetfixer

    Jetfixer Well-Known Member

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    When doing compression test all the plugs should be removed and battery fully charged and throttle wide open (unless carbs are removed) take your readings dry, put a little oil and then take a wet reading . Usually your reading will be slightly higher on the wet reading. Another thing is if readings are low ,you might try another compression gauge most auto parts stores rent them or buy another gauge.
     
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  4. Rooster53

    Rooster53 Well-Known Member Premium Member

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    You can also add that it is best to disconnect the TCI during a compression test, or at the very least you must have a plug in each cap and have the plugs grounded to the engine in order to protect the coils from damage caused by excessive voltage build-up.
     
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  5. Ryengoth

    Ryengoth Active Member Premium Member

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    Get an inline spark checker. 120psi is good. Anything under 100 probably won't ignite.
     
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