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Massive compression drop

Discussion in 'XJ Technical Chat' started by mlwistrom, Jul 17, 2015.

  1. mlwistrom

    mlwistrom Member

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    A few months ago I did a compression test and got about 125-130 psi on each cylinder.
    This week I adjusted my valve clearances and figured that I might as well test compression again when I finished. This time I got around 155 dry and 165 with a squirt of oil on 3 of the cylinders (it was a different tester, probably why it is so much higher). But on the 4th cylinder I got 76 dry and 80 with oil...

    The bike drives just fine. How could I have lost so much compression in a few months?
     
    mlwistrom, Jul 17, 2015
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  2. k-moe

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

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    You may not have. Was the compression tester o-ring making a good seal with the sparkplug hole?
    Was the bike sitting unridden for a period of time? The ring on #4 may just be stuck.
    Can you confirm that valve clearances on #4 really are in spec?

    I would redo the compression test on all four, taking care that the gauge is working properly and all seals are tight.
     
    k-moe, Jul 17, 2015
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  3. mlwistrom

    mlwistrom Member

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    This compression tester has no o-ring. It simply screws into the spark plug hole.
    And yes the clearances are in spec. 7 of my valves were out of spec (by about 0.05 mm each) but I got them all into spec.
     
    mlwistrom, Jul 17, 2015
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  4. k-moe

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

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    But are they still? You have an unexplained compression drop. Valve clearances are easier to check than the condition of the rings and piston. See where I'm going?
     
    k-moe, Jul 17, 2015
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  5. mlwistrom

    mlwistrom Member

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    I guess that I will have to wait for a day off of work to check the clearance again. This bike is my daily as my wife always has the car.

    Question on compression testing. I have always done it with carbs removed. Would it be safe to do with the carbs and fuel tank still on the bike?
     
    mlwistrom, Jul 17, 2015
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  6. k-moe

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

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    Standard practice is to test compression with the machine in running condition (carbs and fuel tank in place), all plugs out, TCI unplugged, and the carbs held at WOT while cranking.
     
    k-moe, Jul 17, 2015
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