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Cylinder Leak-Down Check

Discussion in 'Chainsaw' started by TonyRumore, Dec 5, 2012.

  1. TonyRumore

    TonyRumore AboristSite Guru

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    I've got quite a few saws and have noticed that two saws might blow the same compression numbers on the gauge, but one won't leak down much at all, and just stops the starter rope dead, while the other just blows through like the compression is low. But on the gauge they will read the same. I'm guessing one is leaking down more than the other, even though they both blow the same at high speed. We re-hash the same stuff over and over on this site, but I don't recall ever hearing anyone doing any cylinder leak down check. Any comments on this?

    Tony

    P.S. I'm not talking about a crankcase leak check.....I'm talking about a leak down check above the piston while its approaching TDC.
     
  2. Jacob J.

    Jacob J. Tree Freak

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    Nominally, when you do a compression check, you wait to see how long it holds that compression. Obviously with the two-stroke you're building compression by having the piston gulp air into the cylinder. A traditional "leak-down" check is more of a four-cycle diagnostic.
     
  3. TonyRumore

    TonyRumore AboristSite Guru

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    I understand that, but I hear guys talking about going to buy a used saw and doing a rope-hang compression test to ballpark it. But I've got two identical saws here (actually many more than that) where one blows through the rope-hang test like the compression is low and then blows 170 on the gauge, then I have another that stops the rope dead, but only blows 150 on the gauge.

    Tony
     
  4. one.man.band

    one.man.band ArboristSite Operative

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    usually the saw with the higher exhaust port will have the lower gauge reading, but since these are two identical saws.....who knows!

    -OMB
     
  5. imagineero

    imagineero Addicted to ArboristSite

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    There are more factors at work than just compression with the drop test as has been brought up in past by other members. The condition of crank and con rod bearings, bar mounted on the saw, how clean the inside of the clutch cover is and other factors all play a part. FWIW, I've found older better worn in saws often have higher compression and run with more guts than newer saws of the same model, but the newer saw will do a lot better on the drop test. I think it's because saws do get loose over time, which is a good thing.

    Leak down testing in the sense of using a leak down tester for 4 strokes is pretty much pointless. The whole idea of using a leakdown tester on a 4 stroke is not just to get a percentage reading of how much the cyllinder is leaking, but to find where from - head gasket, rings, inlet or exhaust valve, through the radiator. There is only one place for leakage to occur in a 2 stroke chainsaw - the rings. I'm going to discount the decomp button for the sake of this discussion.

    I don't see what further info you'd be hoping to get from any kind of leak testing. If you have compression, that's a good thing. If you don't, then you're looking at rings, or a piston and rings, or a P&C. Beyond that, all the standard measures for getting more compression - removing base gasket, or a popup, or machining the base of the cylinder.

    Shaun
     
  6. Tzed250

    Tzed250 Account Hold

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    A leak-down test on a two-stroke can provided useful information. The leakage rate will tell the ring condition. If you have a saw that shows 150psi on a compression test but leaks 3% then you know it wil make more compression with new rings. If you have a saw that makes 145psi on compression check, but leaks only 0-1% then you know that is all it is ever going to do. While the cylinder is under pressure you can also spray a little soapy water on the decompression valve and spark plug to check for leakage at those points.
     
  7. albert

    albert Addicted to ArboristSite

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    Swap the starters on the identical saws and see what haappens. Rope diameter and how much is on the pulley will affect pull effort. Also alot of compression testers are piss poor at best. One other thind comes to mind, The easier pulling saw may have a decent amount of cabon build up on the piston and the combustion chamber but has poorer ring seal.
     
  8. Rustynutz

    Rustynutz ArboristSite Operative

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    Probably the best test is cut some wood with it.
    if it starts runs and cuts wood then the rest is all a bit academic.

    happy logging!!!
     
  9. Fred482

    Fred482 ArboristSite Operative

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    I threw this up again, because I couldn't find the original post. I use the leakdown tester on everything, backed up with a compression test. Between the two, I can usually find the general condition of the cylinder. If there's a problem, further diagnosis is done. I hate tearing down stuff that still runs & works........

    View attachment 266322
     
  10. naturelover

    naturelover AboristSite Guru

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    Been thinking o() making a leakdown gauge, never did price one...
     
  11. imagineero

    imagineero Addicted to ArboristSite

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    Ya know, you can get around that by finding an 'f' that someone else has written, select it with the mouse, and press ctrl+c. Now you've got an f in your clipboard. Any time you need one, press ctrl+v. It's not really any more work than typing shift left bracket shift right bracket ;-)

    @tzed, were those percentages arbitrary, or do you really find that good saws have only 1% leak down and replace rings at 3%? How do you hold the piston near tdc?

    Shaun
     
  12. Tzed250

    Tzed250 Account Hold

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    Not so arbitrary. A well built engine will leak less than 1%. More than 2% leakage is cause for concern. After that it's time for a rebuild. Usually I rig something to hold the clutch still to get the job done.
     
  13. z71mike

    z71mike Addicted to ArboristSite

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    Or go to Start and in the Run box, type in On Screen Keyboard. Click away.
     
  14. imagineero

    imagineero Addicted to ArboristSite

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    Maybe the .040 orifice is too large to give any kind of sensitivity to measurements in small engines. For testing of normal 4 strokes I'd consider less than 5% to be excellent, and up to 10% to still be pretty serviceable. A difference of only 1% is a hard thing to measure consistently. It's within the realms of measurement error.
     
  15. Tzed250

    Tzed250 Account Hold

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    When I was first learning about building engines I paid some attention to this guy named Byron Hines. He stated that any competition engine that leaked 1% was junk. The last total rebuild I did on a multi leaked less than 1%, measured with a Snap-on leak down tester.
     
  16. naturelover

    naturelover AboristSite Guru

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    Yea, figured out to copy and paste it yesterday...

    Honestly, It just really never occured to me to do so... :msp_rolleyes:

    But, I can just f away now.... :msp_w00t:
     
  17. lwn9186

    lwn9186 ArboristSite Operative

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    I usually use a deep well socket and a long handled ratchet to hold the flywheel. That way you can check for leakdown throughout the whole power band.
     

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