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Leak down test and compression test procedures

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    Senior Member Mr onetwo's Avatar
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    Leak down test and compression test procedures

    Hey there Boomer, I need help from the Onan guru. I am picking up a very cheap 446 tomorrow.The tractor is totally trashed...broken frame,pump is blown.The engine runs...no smoke or odd noises.My intention is to remove and sell the engine and all related items to a guy who needs it badly for his 446.I am going to keep all the rest of the good stuff for my parts stash.Before I sell this engine I want to make sure of it's condition.I do not want to screw this guy over...wouldn't be right. Anyway, Boomer can you please explain the correct way to do a leak down & compression test on the B series engines, including any special tools or fixtures needed.I just got a new compression test kit.There is nothing about this in any of my Onan or Case engine manuals.I also own a 648 that I want to keep alive forever! Sorry for the length of this ...Thank You very much for your input!!!!

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    Re: Leak down test and compression test procedures

    Quote Originally Posted by Mr onetwo
    Hey there Boomer, I need help from the Onan guru. I am picking up a very cheap 446 tomorrow.The tractor is totally trashed...broken frame,pump is blown.The engine runs...no smoke or odd noises.My intention is to remove and sell the engine and all related items to a guy who needs it badly for his 446.I am going to keep all the rest of the good stuff for my parts stash.Before I sell this engine I want to make sure of it's condition.I do not want to screw this guy over...wouldn't be right. Anyway, Boomer can you please explain the correct way to do a leak down & compression test on the B series engines, including any special tools or fixtures needed.I just got a new compression test kit.There is nothing about this in any of my Onan or Case engine manuals.I also own a 648 that I want to keep alive forever! Sorry for the length of this ...Thank You very much for your input!!!!

    Yes...... your post was much too long. Below is the edited version.


    Hey there Boomer, ...Thank You very much for your input!!!!

    In future, try to be more succinct.






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    Re: Leak down test and compression test procedures

    Onetwo
    With cyl at TDC of compression stroke....lock flywheel in place and with 100+ psi and your gauge zero'd in for what pressure you have connect it to hose screw'd in plug hole and read % of leakage and listen at carb/exhaust/dipstick for air leaking. Repeat for other side. Bob M

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    Senior Member Mr onetwo's Avatar
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    Re: Leak down test and compression test procedures

    Quote Originally Posted by robynrj
    Onetwo
    With cyl at TDC of compression stroke....lock flywheel in place and with 100+ psi and your gauge zero'd in for what pressure you have connect it to hose screw'd in plug hole and read % of leakage and listen at carb/exhaust/dipstick for air leaking. Repeat for other side. Bob M
    Thanks Bob...I am ordering a leakdown kit today from Summit Racing..finally get to use that coupon I have

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    Re: Leak down test and compression test procedures

    Over the years I have performed many leak down tests on aircraft piston engines. The standard for aircraft is 80 PSI input and most of these aircraft engines are long stroke and have cylinder bores over 5". I had a generic small engine service manual here which I cannot seem to locate at the moment but if I recall it recommended a 60 PSI input setting on the leakdown tester for small engines.
    Mad Mackie in CT

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    Re: Leak down test and compression test procedures

    Thanks for the info...I have read in several places that lower PSI will give a more accurate leakdown % on smaller motors.

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    Re: Leak down test and compression test procedures

    So which one of these does everybody prefer?

    What kind of gauge was on the youtube one? 15 PSI seems low compared to all the other tests I've read about.
    I noticed that the air pressure was variable due to a small compressor. Kinda like sandblasting

    I built one similar to the Lovehorsepower one.

    Here's my thought on leak downs. If I didn't have a Leakdown testor but had a small air compressor then I would just hook up the hose to the spark plug Hole and let er buck.

    If your compressor is at 110 lbs and you hear hissing at the Exhaust Pipe, Carb or Dipstick hole then you got a Runner. If there's lots of air coming out then you got a problem.

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    Re: Leak down test and compression test procedures

    I just did some on line research about leakdown testing and I see that there are several different types of testers.
    Single gauge 0-100 PSI calibration
    Single gauge reverse calibration where 100 PSI input reads zero on the gauge, regulated by compresser.
    Dual gauge reverse calibration regulated by compresser.
    Single gauge with a regulator.
    Dual gauge with a regulator.
    Dual gauge with a regulator and an output shutoff.
    The straight and reverse gauges may be calibrated in PSI or percentage with a regulated input of 100 PI. This gives a numerical or percentage of leak rate and a comparison of each cylinder is taken.
    The type of tester that I have used for many years is the dual gauge with regulator and output shutoff valve with 100 PSI max gauge calibration. With this type of tester I regulate it to a low pressure initially and read the leakage rate with the piston at the bottom of the power stroke, shut the output off and then bring the piston around to TDC on the compression stroke, readjust the regulator to a higher pressure usually 60 PSI and check it again. I don't get a percentage of leak rate indication but I'm not concerned with percentage of leak rate but more interested as to where the leaks are and how much. I test both cylinders several times to get a feel for this particular engine and type.
    Leakdown testing is not decisive but rather provides you with the source of a possible internal engine problem. You need to record your findings on paper so they can be looked at and compared carefully. Engines built to different internal tolerances will have different leakage rates however acceptable for that particular engine.
    Bob MacGregor in CT

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    Re: Leak down test and compression test procedures

    A member at the MTF group has an Ingersoll 3016 with an Onan P216 engine that appeared to be running on one cylinder. He replaced the coil, plugs and wires and stil had the same problem. A compression test indicated 105 PSI on one cylinder and 15 PSI on the other cylinder. A leakdown test at this point in time would have shown him that there was a problem with in the intake area on the bad cylinder as compared to the good cylinder. This still leaves several possibilities, valve, valve adjustment, valve seating, or valve seat itself. Removal of the cylinder head and inspection showed that the valve seat was loose allowing the compression to leak by it.
    The grass is drying and this old phart has mowing to do!!!! Seeya!!
    Bob MacGregor in CT

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