Compression numbers low when cold.

Discussion in 'VBoost Room' started by Phil Walker, Jul 14, 2019.

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  1. Jul 14, 2019 #1

    Phil Walker

    Phil Walker

    Phil Walker

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    I just did a compression test on my stock 93 with 50000 miles.
    With engine around air temp 65-70 degrees F.
    From front left counter clockwise. 30,110,70,110.
    After warm up all cylinders right around 120.
    I re-checked the low cold numbers several times removing and rethreading the test hose.
    The reason for the test is that it doesn’t have its normal arm stretching pull between 4000 rpm and redline.
    If I back off the throttle a little it pulls a bit more but surges like it wants to go but never really does.
    This bike hasn’t been ridden a lot the last few years but it idles well and I have cleaned the carbs and synced them recently. I can hear the v-boost cycle when I turn the key on. At lower rpms when you roll the throttle on about halfway it seems right. Also front left spark plug looked lighter colored than the rest.
    What should I make of this?
     
  2. Jul 14, 2019 #2

    one2dmax

    one2dmax

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    When was the last time you checked the valve lash?
     
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  3. Jul 15, 2019 #3

    maleko89

    maleko89

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    +1 to Sean. Also, don't bother on cold test. Only check compression with a fully, warmed up engine. Make sure throttle is fully open as well.

    Mark
     
  4. Jul 15, 2019 #4

    sdt354

    sdt354

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    All the plugs removed.
     
  5. Jul 15, 2019 #5

    Phil Walker

    Phil Walker

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    Now I'm feeling like I didn't have the test hose screwed into the spark plug hole enough on the two low cylinders because after the engine cooled off for a few hours I was still getting readings around 110-120.
    The other running issues are what I'm going to be focusing on.
     
  6. Jul 15, 2019 #6

    Phil Walker

    Phil Walker

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    I've had this bike for the last 15k to 20k miles. I've never checked valve lash.
     
  7. Jul 15, 2019 #7

    jdeitz1979

    jdeitz1979

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    Maybe is carb or air filter related
     
  8. Jul 16, 2019 #8

    sdt354

    sdt354

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    It could certainly be time. And with too little lash the compression starts to leak. The valves don't seal like they should,and the valve timing advances a bit. Exhaust valves will wear tighter. I think the factory recommends 25k for the first check.
     
  9. Jul 17, 2019 #9

    Lil'skeet

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    Try a leak down test. It narrows it down to head gasket, cylinder/ rings wear, exhaust valve not sealing, or intake valve not sealing. Top dead center is critical when checking!
    Saves the guess work that comes with a compression test.
     
  10. Jul 17, 2019 #10

    Lil'skeet

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    And lightly greased the O-rings whether running a compression tester or a leak down tester.
     
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  11. Jul 18, 2019 #11

    sdt354

    sdt354

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    If you choose to use a compression gauge. Shoot some oil into the cylinder, spin the engine and recheck. If the compression reading drastically increases, it temporarily sealed the piston rings, and they are the issue. If it remains the same,valve sealing are the issues. I like the compression test for its simplicity for new wrenchers. The leakdown test is probably a bit more accurate but more detailed in set up. You can hear the leak using the leakdown set-up. I use the leakdown test for before, then after repair results. For example, valve jobs.
     
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  12. Jul 19, 2019 #12

    Lil'skeet

    Lil'skeet

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    A leak down before and after break in is a good way to see how a rebuilt motor breaks in. I keep a record of all my bikes and do it yearly.
     
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