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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 Staff Member

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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 ArboristSite 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 Staff Member

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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. 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
     
  12. 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
     
  13. 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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  14. 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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  15. naturelover

    naturelover ArboristSite 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
     
  16. 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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  17. snowd

    snowd ArboristSite Lurker

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    After reading these this morning, I did a compression test on my 260 & 290 today. Both saws are cold and have not been started in a few days. The 260 is at 155 psi and the 290 is at 180 psi. Gauge has a 12" hose and a scrader valve in the end of it.

    IMG_1199.JPG IMG_1201.JPG IMG_1202.JPG
     
  18. naturelover

    naturelover ArboristSite Guru

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    If you want to do a little test, remove the Schraeder valve and pull it over. Most likely you'll be able to feel the difference.


    Sent from my iPhone 5 using Tapatalk
     
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  19. snowd

    snowd ArboristSite Lurker

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    Ok, I fell for it. Ha Ha. I went out and took the schrader valve out and pulled it over. After a few pulls looked at the gauge puzzled because it read zero, duh, no valve to hold the pressure in it. Thanks for the laugh. And yes it pulls over like the plug is out of it.
     
  20. axe2fall

    axe2fall Hound runner

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    I get it now, thanks. I think I was hung up on the multiple pulls to build compression thing.
    I did know the difference between the Schrader and the release but I assumed they were together. I see now that the valve on mine is at the bottom of the hose
     

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