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Saw compression test?

Discussion in 'Chainsaw' started by Coldfront, Feb 3, 2014.

  1. Coldfront

    Coldfront Addicted to ArboristSite

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    When you guys talk about compression are you testing with the engine warmed up or cold?
    I been reading some threads on here saying they are getting 175psi on some saws. I bought a new compression tester and did my 2007 372XP cold and only got 130 psi max. That must be getting on low side? The saw still starts and runs good. I'm just wondering how bad is 130psi from a 8 years old 372xp?
     
  2. CR500

    CR500 Diesel, Snowmobile, & Chainsaw Technician

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    I do it with a cold engine Full throttle. the high compression number you are seeing may be coming from "ported saws"
     
  3. lone wolf

    lone wolf Tree Freak

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    Did you pull it till the gauge stopped rising about 10 times? !30 really is not good but will run ok. 150 160 plus is ideal.
     
  4. Marshy

    Marshy Addicted to ArboristSite

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    As stated, hold throttle wide open.
     
  5. Coldfront

    Coldfront Addicted to ArboristSite

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    I had throttle wide open and pulled it about 5 or 6 times it don't go higher than about 130. It hits 130 on about the 4th pull.
     
  6. maico490

    maico490 AboristSite Guru

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    Makes no difference as air will be drawn through the exhaust.
    The main thing with compression testing small engines is to make sure the tester has a Schrader valve in the tip not somewhere up near the gauge.
     
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  7. axe2fall

    axe2fall Hound runner

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    I know this is common knowledge on this site and others, but why?
    Won't the compression equalize throughout the hose if the engine is cycled enough?
    Just asking because I have the wrong kind and hoping someone will tell me it's good :cool:
     
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  8. Coldfront

    Coldfront Addicted to ArboristSite

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    Yea mine has the schrader valve in the end that screws in, and also has a compression release up by the gauge. I have read a couple different places that all say have the throttle open. I don't really see what that does either, maybe makes it a little easier to pull, but if you keep pulling until the gauge won't go any higher should be the same closed or open.
     
  9. watsonr

    watsonr Aftermarket Parts Pusher

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    When the piston is down, air flows into the combustion chamber thru the muffler. You can't spin it fast enough to prevent it from happening, holding the throttle won't matter.
     
  10. Marshy

    Marshy Addicted to ArboristSite

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    I stand corrected then. I know with 4 strokes it's important and I thought it applied to 2-strokes too.
     
  11. Stephen C.

    Stephen C. Addicted to ArboristSite

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    no it won't. The volume of the hose becomes part of the volume of the combustion chamber lowering the compression ratio. The smaller the engine the larger the effect.
    GAGE.jpg
     
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  12. Stephen C.

    Stephen C. Addicted to ArboristSite

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    I would be surprised if it maxes out in 4 pulls unless it has an extremely short hose or none at all. If you watch it you will see diminishing returns, 80# on the first pull 30# more on the second 15# more on the third 7# more on the fourth 3# more on the fifth 1 1/2# on the sixth 3/4# on the 7th, 3/8# on the 8th and on and on . The length of the hose and the size of the piston will alter these values but Until you get to around 8 pulls you have not got all there is to get with a hose length of around 12 inches.
    3400-2.JPG
    4200.JPG
     
  13. axe2fall

    axe2fall Hound runner

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    I see what you are saying and how the hose would change the compression ratio. But I can't grasp how it changes compression.
    I guess I am looking at compression as simply pressure. And a compression gauge as a tool to measure the pressure the engine can build before any additional pressure blows by the rings. . . . Like the engine is a pump and it just has to cycle more times in order to bring the larger volume of air to the same pressure.
    I really am trying to understand this and I appreciate your patience
     
  14. axe2fall

    axe2fall Hound runner

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    In thinking about my own "blows by the rings" statement above . . . Could timing be a factor? . . . . As in the time it takes to push the air up the length of hose allows the piston to come down enough to release some compression through the exhaust before it passes the Schrader valve ???
    Sorry if I am beating a dead horse
     
  15. krushing73

    krushing73 ArboristSite Member

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    I see about the same thing on my saws. In my gauge 130 is about as good as it gets. Always wondered the same thing. I just use the 125-130 as a bench mark. Works fine unless you are taking a picture to put on eBay. I usually change rings at less then 100 on my gauge. I have Schrader valve.
     
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  16. nmurph

    nmurph ArboristSite.com Sponsor

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    130psi isn't a bad pressure for a 372. It is a little low, maybe time for a re-ring. But they typically run in the 145psi range.

    Most modern saws run 150-165psi- new, broken in.

    Some vintage saws run 120psi- new, broken in.

    Partner 5000+ run 185-190psi.

    Test cold, but the saw should have run in last few days so that it will have some oil film on the cylinder. That's not the same as oiling the cylinder to give a falsely high reading
     
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  17. naturelover

    naturelover AboristSite Guru

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    You are increasing the area in which the piston has to compress the air, and it only gets one shot at it, it doesn't build upon or hold that pressure on each stroke.


    Sent from my iPhone 5 using Tapatalk
     
  18. Stephen C.

    Stephen C. Addicted to ArboristSite

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    It isn't pumping a larger volume of air. It is pumping the same volume of air into a larger space. If it is a 50cc saw it is pushing 50cc of air into the combustion chamber, that is all it can ever push. If the chamber becomes larger the achieved pressure drops
    if the Schrader valve is located at the spark plug hole the hose wont have an effect on the final reading just a longer hose will require more pulls on the rope. The ultimate obtainable compression is determined by the piston displacement and the combustion chamber size. With the check valve at the spark plug hole the hose will retain pressure with every pull until it is equal to the pressure in the combustion chamber. If the check valve is at the other end of the hose the hose becomes part of the combustion chamber volume. Since the piston displaces the same amount of air as it did before and the combustion chamber volume has doubled or tripled the the result will be 1/2 to 1/3 of what the actual pressure in the combustion chamber would be with a spark plug installed.
    I have never seen a gage built that way but I am told they exist. Some people confuse the Schrader valve with the pressure release valve. 2 different animals.
     
  19. Stephen C.

    Stephen C. Addicted to ArboristSite

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    Yes my Homelite 923, Partner P-85 and Poulan 4200 all pump over 180 tested warm. I start them and run just long enough the get things lubricated, pull the plug and test.
     
  20. Coldfront

    Coldfront Addicted to ArboristSite

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    I know on most other engines beside single cylinder engines the actual number on the gauge is not as important as the difference in pressure between 2 cylinders. Most times on a 2 stroke you are using a compression test to find a serious problem, hole in piston, broken or badly scored rings. If the thing runs and starts ok I wouldn't be to worried about the actual number, unless it was really low. Then a squirt of oil in the cylinder and all of a sudden a lot higher reading would suggest bad rings.
    I mean when does a person usually do a compression test? When the thing won't start.
     
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