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Thread: Leak down test

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    Red Devil's Avatar
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    Leak down test

    Getting ready to do a leak down test just for the hell of it. For anyone who has done one themself, can you verify these few things? The manual has a few steps such as warming up the engine, removing the valve cover, and positioning the cylinder at the very top. Can't you just turn the shaft until the piston reaches the top? Is this really necessary or can you simply insert the gauge and then supply the air. I also didn't see how much psi to load the cylinder up to.


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    AaronC's Avatar
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    Quote Originally Posted by Red Devil View Post
    Getting ready to do a leak down test just for the hell of it. For anyone who has done one themself, can you verify these few things? The manual has a few steps such as warming up the engine, removing the valve cover, and positioning the cylinder at the very top. Can't you just turn the shaft until the piston reaches the top? Is this really necessary or can you simply insert the gauge and then supply the air. I also didn't see how much psi to load the cylinder up to.
    It's best to warm up the motor because you're trying to simulate a motor running. A warm motor under pressure seals better and will yield more accurate figures. You need to put the piston to TDC on the FIRING phase or power stroke and not the OVERLAP phase. The valves need to be shut 100%. Hitting the cylinders with 100 psi is the easiest way to figure out leak %. if you get 95 then you have 5 psi leak down. Each psi equals 1%You can hit it with more but just need to do the math to figure %

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    Red Devil's Avatar
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    Thanks Aaron,

    Now, about checking that TDC. Remove the valve cover and check the rocker arms or just do a visual of the piston through the plug hole. Geeze, or does this mean that I have to remove the exhaust manifold? Would there be an indication of the valves open or closed while doing the test?

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    Moderator two2curupt's Avatar
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    Are you getting alot of blow by Jay??? Or losing power??

    your ski sounds like it is flyin!!! you have 100 more rpm's to play with!! you should hit some good speeds when you pitch the prop for those 100 rpm's
    But low 76's are not bad!!!!

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    Red Devil's Avatar
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    Quote Originally Posted by two2curupt View Post
    Are you getting alot of blow by Jay??? Or losing power??

    your ski sounds like it is flyin!!! you have 100 more rpm's to play with!! you should hit some good speeds when you pitch the prop for those 100 rpm's
    But low 76's are not bad!!!!
    Mike,

    I think that 3 to 4 ounces is a lot of blow but maybe not. Yes, I am quite pleased that the GTX is running 76's, it is flying, and I am not losing any power at all, but, I find it hard to believe that my rings have not seated yet either. This blow by is happening for a reason and I want to find the culprit.... Once I do the leak down, and confirm the rings are fine, then I'll accept the blow by......Like I said, if this amount of blow by is normal, I'll go with it. You know me Mike, I don't leave any stones unturned..

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    Moderator two2curupt's Avatar
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    Is Jerry not running the same setup as you?? Accept in his wifes RXP??

    I am not sure if that is normal with a B-Kit?? I jut get a slight oil smoke from mine...

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    Red Devil's Avatar
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    Quote Originally Posted by two2curupt View Post
    Is Jerry not running the same setup as you?? Accept in his wifes RXP??

    I am not sure if that is normal with a B-Kit?? I jut get a slight oil smoke from mine...
    Sort of, but not totally. Jerry is running EMS with 6 stock injectors.

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    captain obvious Lurker's Avatar
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    You can also check TDC on the compression stroke by leaving the head on take the spark plug out and put your thumb over the hole. The most pressure will be felt when cranking over and on the compression stroke. This is easier to show then explain.

    You really should take the head off because if the piston is anywhere except TDC it will crank over when you apply pressure.

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    GTXX's Avatar
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    Quote Originally Posted by _Lurker_ View Post
    You can also check TDC on the compression stroke by leaving the head on take the spark plug out and put your thumb over the hole. The most pressure will be felt when cranking over and on the compression stroke. This is easier to show then explain.

    You really should take the head off because if the piston is anywhere except TDC it will crank over when you apply pressure.
    how do you do a leak down test with the head off?

  10. #10
    TRIPLE PIPES ARE 2-STROKE TURBOS allcool's Avatar
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    Quote Originally Posted by Red Devil View Post
    Thanks Aaron,

    Now, about checking that TDC. Remove the valve cover and check the rocker arms or just do a visual of the piston through the plug hole. Geeze, or does this mean that I have to remove the exhaust manifold? Would there be an indication of the valves open or closed while doing the test?
    I am not a SD guy but know engines and have done many leak downs on various engines.

    Hope this helps.

    The reason to take off the valve cover is it is the easy way to confirm all valves are closed with piston @ tdc.... then move to the next cyl and do the same. When rockers are loose to the touch that means the valve has no pressure on it from the cam lobe, so valve is closed. This is the valve position most engines get valve lash adjustments done at.

    allcool

    I think the other guy means valve cover not head.

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