Generatory cylinder compression
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Generatory cylinder compression

  1. #1
    Islander Transient Chipster27's Avatar
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    Default Generatory cylinder compression

    OK, here's an odd one. Does anyone have any idea what the compression should be in a generator engine?
    Model: Yamaha YG2800i
    Engine size 171 cc

    I ran a compression test and it was about 50 PSI. This seems incredibly low as most engines I have tested are closer to 150+ PSI. But I thought that due to the smaller displacement, fact that it's not a high compression engine and it's pull start rather than electric, that the compression might be lower. But 50 seems too low.

    Any ideas???

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  3. #2
    Engine junkie
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    The unit most likely has a decompression mechanism used for start up, once fired the decompression mechanism disengages.
    To answer the question about compression, it takes a minimum of 90psi for combustion to "correctly" take place.
    You may want to do a valve adjustment if not already done, this may clear up some issues too.
    Life's journey is not to arrive at the grave safely in well preserved body, but rather to skid in sideways, totally worn out, shouting"....holy shit.... what a ride!"

  4. #3
    Islander Transient Chipster27's Avatar
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    Quote Originally Posted by SRP1 View Post
    The unit most likely has a decompression mechanism used for start up, once fired the decompression mechanism disengages.
    To answer the question about compression, it takes a minimum of 90psi for combustion to "correctly" take place.
    You may want to do a valve adjustment if not already done, this may clear up some issues too.
    Thanks, yes it does have a decompression mechanism and the genny runs fine, although it's louder than I would expect it to be. I do valve adjustments once a year and an oil change twice a season. Just wondering what the correct reading should be. The genny is 6 years old and for the past 3 years I've used it alot so I'm trying to determine if it's time to hone the cylinder and put some new rings in.

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    steelcomp was here
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    If it aint broke...
    If God is your co-pilot, change seats!
    Acts 2:38, the perfect answer to the perfect question.

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    Senior Member Brendellajet's Avatar
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    Quote Originally Posted by steelcomp View Post
    If it aint broke...
    Then it might break pretty soon, better fix it.

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    Quote Originally Posted by steelcomp View Post
    If it aint broke...
    Guess I'll have to break it

  9. #7
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    If its not smoking, showing signs of oil fouling or excessive carbon build up on the plug then run it.
    You can do a leak down test on the cylinder, that will give you better feedback than a compression test.
    Life's journey is not to arrive at the grave safely in well preserved body, but rather to skid in sideways, totally worn out, shouting"....holy shit.... what a ride!"

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