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How much compression in to low?

Discussion in 'XJ Technical Chat' started by zrttodd, May 27, 2009.

  1. zrttodd

    zrttodd Member

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    I have a 1982 xj 750 that has 125 psi on 2 cylinders and 115 on the other 2.It has 52,000 miles on it and needs carb manifolds and throttle shaft seals.I don't want to put the money into it if it needs a motor rebuild.
     
  2. bluepotpie

    bluepotpie Member

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    i'd run with it.
     
  3. zrttodd

    zrttodd Member

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    I meant how much compression IS to low.Also this was done on a cold motor with the carbs wide open.
     
  4. RickCoMatic

    RickCoMatic Well-Known Member

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    Too low is when the Cylinder fails to fire, the Blowby rushes into the Crankcase and does not oppose the Inertia of the Piston that reaches Top Dead Center and keeps on going a few thousandths passed the stroke, separating from the Connecting Rod which continues without the Piston attached.

    Not good.
     
  5. zrttodd

    zrttodd Member

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    Thanks RickCo, but it's not quite that low yet.
     
  6. MiCarl

    MiCarl Active Member

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    What are your wet numbers?
     
  7. zrttodd

    zrttodd Member

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    How do you do a wet check?
     
  8. bigfitz52

    bigfitz52 Well-Known Member Premium Member

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    Those are kind of low, however the difference is not TOO great between them. Remember overall reading isn't as important as the differential readings.

    Two things: Get a "second opinion." As in, use a different compression gauge. Gauges vary, and there's no sense panicking if you have a soggy gauge.

    Do a "wet" reading as Carl suggested, and post the results.

    A valve job is not the same as a "motor rebuild" and if you've gotten 52K without one so far you're doing great. My Norton (granted, more primitive technology) has had two valve jobs and it has 45K on it.
     
  9. MiCarl

    MiCarl Active Member

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    Put a small quantity of oil (teaspoon or so) in the hole before you check the compression. This will seal the rings and if the numbers come up significantly then you know that is where the problem is.

    If the numbers don't come up then you know to think valves or head gasket.
     
  10. bluepotpie

    bluepotpie Member

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    same as you did, only with a tablespoon or two of oil in each cylinder. this helps seal the rings to see if your issue is rings or valves.
     
  11. DaveXJ

    DaveXJ Member

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    First thing i would do, is get another compression gauge and try. Have the bike warmed up some so that you can still touch the motor without getting burned and then do the check while holding throttle open. The differences between my old gauge and the new one i bought was about 20PSI !!
     

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