Cold Compression Test

Discussion in '1st & 2nd Generation 1983-1989' started by Michael E, Jul 14, 2010.

  1. Michael E

    Michael E New Member

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    I finally bought a compression gauge and I went to do my first compression test last night. Unfortunately I forgot to warm up the bike before I took everything apart. I did the test anyway and here are the readings:

    1 - 120
    2 - 130
    3 - 120
    4 - 125

    I am happy with the consistency, but concerned about the value. If I did the test hot, how much would you estimate these number would increase? Should they get into the minimum spec range (140+ I believe)?
     
  2. sherkanner

    sherkanner New Member

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    You can try adding a spoonette (little spoon) of oil in each cylinder when you retest the compressions again.
    If it rises a lot, your segmentation needs to be changed. If it is only marginally changing, it should be good.

    I don't know if by hot you will have a lot of differences, but the oil will be more fluid, so in my opinion, it should go down. But I'm not yet an expert on this topic.
     
  3. tinkerinWstuff

    tinkerinWstuff Administrator Staff Member

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    Those numbers look good. wouldn't give it a second thought.
     
  4. slowbird

    slowbird Member

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    Aren't they a tad low?

    Mine were:

    1- 110
    2- 140-150
    3- 140-150
    4- 140-150
     
  5. invisible cities

    invisible cities New Member

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    My Service Manual notes: 171 +/- 28psi
     
  6. tinkerinWstuff

    tinkerinWstuff Administrator Staff Member

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    his test was cold. A warm motor will go up. The values are also for sea level so altitude can play into it.

    Then, ultimately, what are you going to do about it if it's below the "minimum" spec? re-ring the motor? Waste of money. You won't know the difference between between 120psi and 140psi. Ride that thing.

    I think the minimum spec has more to do with the motor being new and you trying to deal with a dealer service department. After that, the minimum is equal to what you are willing to live with. As long as they are within 10% across the board, don't sweat a thing.

    My 500 had 105-110 cold and at 5000ft above sea level. The thing rode fine. Had a little bit of blow by that would cause the oil level to drop thru the crank case vent. Long highway runs required oil level supervision and top off every couple hundred miles.
     
  7. slowbird

    slowbird Member

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    Good point. Well put.
     
  8. invisible cities

    invisible cities New Member

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    +1, Always good to have you around Tink. Sure you don't want another First Gen?
     
  9. tinkerinWstuff

    tinkerinWstuff Administrator Staff Member

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    space in the garage and money.....

    Love the looks of a good 1st gen.
     
  10. tinkerinWstuff

    tinkerinWstuff Administrator Staff Member

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    There isn't a motor out there who's compression reading won't go up with oil poured in the cylinder. This test is more telling when you have one cylinder that's significantly lower than the rest and you want a clue whether it's a ring or valve problem. Adding oil to the cylinder and the pressure doesn't increase means there is a valve problem.

    I believe it's reasonable to expect about a 10psi increase on a hot motor. This has nothing to do with the oil but with expansion of metal components. The rings and valves will seat and seal better warm as they expand into their normal operating position that they've "broken in" to.

    Cheers :tea:
     
  11. invisible cities

    invisible cities New Member

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    Hear, hear! kR's and VF500F's two of my favorite designs.

    I believe that you have some good karma points with the VF gods after getting your old steed up and running. I think that they would smile upon you with the gift of a gremlin free First Gen, should room open up in the stable.
     
  12. Michael E

    Michael E New Member

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    I suspect that the 130 cylinder might be the one that a previous owner bent the connecting rod on. The owner after replaced a connecting rod and piston from a donor motor (which I have). That helps confirm my suspicion that the donor motor was in better shape than the one being repaired. Cams, clutch and some other parts were swapped over too. It is a mystery why he just didn't use the donor instead of the other motor.....
     
  13. tinkerinWstuff

    tinkerinWstuff Administrator Staff Member

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    the 130 cylinder is right next to the 125 cylinder. Thats within 3.9% of each other - not to mention cold. I really wouldn't agree that it confirms or says anything.

    You're in great shape :thumbsup:
     
  14. Michael E

    Michael E New Member

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    I said "helps" confirm. As you say, can't be conclusive. A mystery still, this bike is....(insert yoda voice).
     
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