Help, cyl compression question

Discussion in 'Sportster Models' started by 67hat34c, Aug 7, 2010.

  1. 67hat34c

    67hat34c Active Member

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    2008 1200c.

    Checked compression with 2 guages. one says 180 other says 190

    put in .5oz of oil and compression increases to 240 on second gauge.

    Both cylinders seem close.

    shop book says normal is 200 to 225
     
    Last edited: Aug 7, 2010
  2. Hoople

    Hoople Account Removed

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    And on the 1st gauge it remained at 180? What happened to the 1st gauge?

    Did you run the test with the throttle body or carb butterfly Wide Open?

    Was the test performed with engine warm?
     
  3. 67hat34c

    67hat34c Active Member

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    checked with second gauge with the oil, cant remember the exact numbers but think it remained 10lb different

    Engine at running temp. Forgot to open throttle but did let it turn over 7 times so top number should not be any different just the initial numbers.

    Hoop my bike is the one having the tuning issues that you gave me advice on.
    Just got it back from HD yesterday and they said they did more tune. Does run different and possibly a little better at faster speeds but at idle it is rough and at low speeds in low gears it is very rough. 15mph in 1st gear is very jerky. Still misses after sitting at idle for 70 seconds.

    As you recommended I am now checking compression. Will do vacuum test also.

    Book says 200 to 225 is normal warm compression. Either gauge is below this. Bike now has 16k on it.
     
  4. Hoople

    Hoople Account Removed

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    That is a Big lift in compression from injecting some oil. Compression coming up that much seems to indicate the rings are not sealing like they should. A cylinder leakdown test would confirm the results. Both tests should be run before coming to a conclusion. Lower than normal compression in BOTH cylinders really should not make the bike run bad.

    Remember injecting .5 oz of oil will decrease combustion area by 15cc so that in it-self will increase compression.
     
  5. fin_676

    fin_676 Experienced Member Staff Member Moderator Contributor

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    Throttle has to be open
    if the throttle butterfly is not open then you wont get sufficient air into the cylinders to get a true reading
    the purpose of the throttle butterfly while its efi or carbed is to control the flow of air into the cylinders
    compressing a vacuum gives no reading try the compression test with a full flow of air IE throttle open reading will be higher

    Brian
     
  6. glider

    glider Veteran Member

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  7. NEWHD74FAN

    NEWHD74FAN Experienced Member Retired Moderators

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    The key here is the engine when operated from the starter is an air pump, if you restrict the intake or exhaust your compression reading will be affected. The 180 psi or more being read is within the tolerance one would expect. If you use push on guage, it is hard to hold down that "chuffing" cylinder without leakage around that sparkplug hole rubber nozzle seal.

    An electric motor functions in "reverse" also...if you spin the motor mechanically, electrical voltage will be seen on the electrical connections (basically the principle behind an electrical generator).
     
  8. 67hat34c

    67hat34c Active Member

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    Not push in gauge, New guage read 180 old one read 190, now that I think of it the old one always read 10lb high.

    Shop manual says 200 to 225 for the 1200. So I am a little confused with the 150 to 180 range being normal. Book does say that numbers below 150 are bad.
     
  9. glider

    glider Veteran Member

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    150-180 is considered a good working range. You also must discount for carbon build up and the compression figures are for a new motor. Usage in the motor will lower that figure from wear but when you get into the lower 100"s that's when you have a problem. It is also necessary to open the throttle fully and lift the slide in a CV carb to get accurate numbers.
     
  10. 67hat34c

    67hat34c Active Member

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    Raining this morning so I will warm it up and ck again with TB open.