John Deere 420 oil in aircleaner
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Thread: John Deere 420 oil in aircleaner

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    John Deere 420 oil in aircleaner

    I have a John Deere 420 17xx hours that smokes after it is used for over an hour. I found oil in the aircleaner I suspect that is what is causing the smoke. The plugs look very good. No oil on them. Compression is 100# on both cylinders. Other than the oil issue it works great.

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    bloodrunsgreen's Avatar
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    When was the last time you adjusted the valves on the engine? I suspect they may need to be adjusted to keep the pressure down in the block. I wonder what style of breather you have on it?

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    Thanks for the reply. I have never adjusted the valves. I will check the breather as well. Is 100# compression normal on a Onan motor? Not sure what the compression should be. Thanks

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    Quote Originally Posted by jackson63 View Post
    Thanks for the reply. I have never adjusted the valves. I will check the breather as well. Is 100# compression normal on a Onan motor? Not sure what the compression should be. Thanks
    Is it a b or p series?

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    It has a B48G motor.

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    Quote Originally Posted by jackson63 View Post
    It has a B48G motor.
    I think the minimum is 100, with a 10psi maximum difference between the cylinders.

    A valve adjustment is a good idea, so is cleaning the breather while you’re in there.
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    To All Concerned:

    Spec's for both the Onan B48G and P220G engines call for 75 PSI minimum on the low side to 115 PSI on the high side for compression. You are comfortably in the mid range for either type of these engines................Hope this helps you......................Gary
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    According to the manual TM1950 the minimum pressure is 100 psi for all B type engines.

    Click image for larger version. 

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    Onan Compression Specs

    Gentlemen;

    Really not worth arguing the point. Evidently different issues of Onan service manuals call out different specs (see attached sheets, each taken from Onan service manuals). The most important figure would be the difference from side to side ...........Gary
    Attached Files Attached Files
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    Sounds like piston rings could be getting tired.
    1984 JD 316 w/ cat 0 3-pt hitch and 2000 rpm pto, 48" tiller, 49" snowblower, 42" belly mower
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