airhead cyl compression question

Discussion in 'Airheads' started by The Raven, Apr 10, 2011.

  1. The Raven

    The Raven Banned

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    This question is in my thread, but figured I would toss it out to all to get some more responses.

    My r80 when cold has 80lbs compression in the left cylinder and will not immediately fire. My right cyl when cold is 115lbs

    The valves are in tune at last check and the tapping sounds fine when started.

    Today I tossed in some marvels mystery oil and let it sit a few hours. Blasted it out, tossed the plug in and she started fine. Well with a bunch of smoke :lol3

    I let it run a couple minutes to burn the oil out then did another test. Left cylinder went from 80 to 122, right cyl went from 115 to 125.

    Thoughts?

    I'm guess a stuck and rusty ring from winter condensation?
    #1
  2. wirewrkr

    wirewrkr the thread-killer

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    carbs off, or at least sliders held up and open?
    if so, and oil brings up the readings that much, RINGS.
    but even still it is a little low.
    How many miles on the valve job?
    What's the leak down test tell you?
    It's actually the better, more important of the two tests.
    #2
  3. The Raven

    The Raven Banned

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    Oil actually brought it to about 165 from 80 :eek1

    After I burnt the oil out and got it to stop smoking it dropped to the 120s and inline with the RHS.

    Not sure how to do a leak down test?

    10k on the valves
    #3
  4. The Raven

    The Raven Banned

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    OK, did the compression test with the cyl at 90 degrees F, carbs off, both plugs out and grounded.

    Results:

    L&R 132lbs of compression.

    This still does not explain the the low compression while cold. Will be interesting to see the result tomorrow after it's stone cold.
    #4
  5. herman maire

    herman maire Been here awhile

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    My r75/7 after not running for 20+ years before I got it, I tested it cold since its not running yet at there even at 90psi. its on the low side but I hope it increases like your bike did.... I dont feel like another engine project.

    Maybe a good highway run will even your compression out.
    #5
  6. Big Bamboo

    Big Bamboo Aircooled & Sunbaked

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    Compression should be checked when the engine is hot, not "stone cold".
    #6
  7. Wirespokes

    Wirespokes Beemerholics Anonymous

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    The G/S doesn't have a high compression ratio, so 130 doesn't sound bad to me. The more important thing is comparable figures side to side, which you now have.

    My guess is there was carbon build up you blew out, that was sticking the rings.
    #7
  8. Xcuvator

    Xcuvator Justa Venturer

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    Another thing to consider is that when you introduce a bunch of extra oil to the cylinders, it will take a good brisk ride to burn/blow it out of the ring grooves. I have heard up to 100 miles. I think that is a little excessive, but even if it doesn't take near that long, a good hard ride is needed.
    #8
  9. jimbee

    jimbee Airhead Intermediate

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    If you don't mind, next time you're doing this, could you please take a picture of how you ground the plugs? This is something that I haven't got my head around yet. many thanks
    #9
  10. The Raven

    The Raven Banned

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    No need for a photo.....take plugs out, insert into wires, then rest plug on the head, ground accomplished.
    #10
  11. mykill

    mykill odd

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    My Norton came to me after sitting for 10 years. I did an initial compression test and had 80# in each. Fired it and rode it a bit and have had 128# in each ever since. Your rings just need to get friendly with the barrels again.
    Leakdown is the best diagnostic test.
    #11
  12. bpeckm

    bpeckm Grin!

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    I'll second that! I would not obsess over it... though I know how we feel after a winter's wait....:evil
    #12
  13. One Less Harley

    One Less Harley OH.THAT'S GONNA HURT

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    just pull the hot wire from the coil, no need to worry about loosing the ground
    #13
  14. jimbee

    jimbee Airhead Intermediate

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    Hey neat idea.

    The reason I was asking because I've always been curious about snowbums "other" method for carb sychronization, but after reviewing his tome, I see now that it does not involve removing the plugs.
    #14
  15. One Less Harley

    One Less Harley OH.THAT'S GONNA HURT

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    Of course you know I'm not referring to the high tension lead from coil to spark plug, but the actual power leads to the coil!! Also for compression test not synch.
    #15
  16. jimbee

    jimbee Airhead Intermediate

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    got it and got it - thanks for checking
    #16
  17. batoutoflahonda

    batoutoflahonda Long timer

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    So mystery oil really works? I gotta get me some of that.:lol3
    #17
  18. Solo Lobo

    Solo Lobo airhead or nothing

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    Here'e my G/S... who knows if the gauge is any good or not...

    [​IMG]
    #18
  19. One Less Harley

    One Less Harley OH.THAT'S GONNA HURT

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    Oh come now, your pressures not that high, you were blowing in it right!!!!
    #19
  20. Solo Lobo

    Solo Lobo airhead or nothing

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    LOL! I was totally shocked... no clue how it happend, but the other side read within 2 lbs... the carbs were off.
    #20