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Chainsaw Compression Testing

Discussion in 'Firewood, Heating and Wood Burning Equipment' started by hlhopp, Feb 1, 2011.

  1. hlhopp

    hlhopp ArboristSite Lurker

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    I've got 6 Stihl chainsaws from 009 to 036 and they all have about 80 to 100 p.s i. compression according to my testing. From reports I've been reading, and in ads, like e-bay ads, they report 150 and 160 p.s.i. compression rating. Am I doing it wrong? I tape throttle open pull cord about 10 times and watch guage. I'm ready to get a new compression tester.
     
  2. CTYank

    CTYank Peripatetic Sawyer

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    1- try doing it with engine warmed up.
    2- make sure compression gauge fitting is screwed into plug hole so it seals.
    3- you need a schrader-valve at above fitting with a purposely weak spring.

    The actual pressure gauge could be n.g.; try borrowing another tester?
     
  3. loadthestove

    loadthestove AboristSite Guru

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    your checking it right,but I'm not sure what the numbers should be.
     
  4. hlhopp

    hlhopp ArboristSite Lurker

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    Thanks CTYank,
    That was my first forum post ever and I was surprised to have a reply so soon!
    The compression tester is a quality unit, I believe, but I am doubting it. Guess I'll borrow or buy another one to try. My 036 has a new cylinder kit I put on it so I was expecting over 100 p.s.i. but nope, it had about same as old cylinder with some scoring, about 90????? My saws all run great so I know they have good compression, I'd just like to know what compression I should expect.
     
    logging22 likes this.
  5. Wood Doctor

    Wood Doctor Edwin

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    Look for 150 PSI

    That's about the average. Anywhere from 140 to 150 is pretty good. The saw will probably run if compression is as low as 90 to 100 psi, but reports have varied on this issue because of timing. Spark timing is very important--probably more important than compression. If the timing is off, the compression is almost meaningless.
     
  6. Hedgerow

    Hedgerow HACK

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    Unlike a 4-stroke, I don't think the throttle needs to be wide open. This is according to the chainsaw doc. And it makes sense. Did you crank em' real brisk? Or just roll em' over? Because they should be reading over 100 lbs even if well used. There are some guys on this site that know about every stat. on those saws. I figure they'll be along shortly.
     
  7. TreePointer

    TreePointer Addicted to ArboristSite

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    "They'll be along shortly..." especially if the OP asks this question in the Chainsaw Forum.
     
  8. avalancher

    avalancher Arboristsite Raconteur

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    I seriously doubt that you have compression as low as 80lbs and the saw is still running good.Most saws dont run very good if at all below 100.My old 371 had a 100lbs compression before the rebuild,and while it did start you had to prime the engine with fuel in the spark plug hole for it to fire, it didnt have the necessary vacumn to pull the fuel into the cylinder at 100 psi.And needless to say,it didnt have the power to pull even a 24 inch bar. After the rebuild, it ran at 150lbs and ran like a champ,even pulling a 32 inch bar with semi skip.

    I would first off make sure your o-ring on the end of the tester is sealing off where it screws into the cylinder.Put some soapy water around the end and look for bubbles when pulling the starter cord.If you have bubbles, replace the o-ring and try again.If that checks out, try another compression tester.Most Autozone type parts houses will loan you one for free.
     
  9. CWME

    CWME AboristSite Guru

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    I would be more worried about what caused the scoring on the old piston than about PSI numbers at this point. Did you find the root cause of the issue before you put a new P&C in?

    My MS440 had 150 PSI cold if memory serves with the new Big Bore P&C kit I installed. Haven't checked it since it was new, don't see the need if it is running and cutting my firewood.
     
  10. hlhopp

    hlhopp ArboristSite Lurker

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    Thanks for all the help on my compression testing issue. The reason my 036 had a scored cylinder is the previous owner accidentally ran straight gas in it and siezed it up. He GAVE it to me and bought a new one when he found out how much a shop wanted to rebuild it. I just took out the piston, freed up the stuck rings filed out the ring grooves, hand sanded the piston and cylinder, cleaned and oiled and reassembled and it ran great. I ran it for a couple years with a 20 inch bar, but I just got a 24 inch bar so put a new cylinder kit on it for more power. According to my testing, the compression is the same (95 p.s.i.) with the new stuff as it was before . I think my tester must have something wrong with it letting it only go up to around 90 no matte what the compression is. I like the suggestion of Autozone loaner. I think I'll go there tomorrow and get one and try it. Thanks everyone!
    One question, how do you put the stuff like what saws you have on posts?
     
  11. Wood Doctor

    Wood Doctor Edwin

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    The chainsaw list is part of your signature. It remains a constant. The avatar picture of my truck is done by reducing a picture down to the minimum size allowed and then linking to the picture at Photobucket, the site where I post lots of pictures. At Photobucket, I believe you can shrink any picture as need be, so no other software is required.

    Settings --> Edit Signature
     
  12. hlhopp

    hlhopp ArboristSite Lurker

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    I have the compression testing problem solved! First I got a loaner tester from Auto Zone and it didn't work any better than mine!!!! I then went to good old Harbor Freight and bought one. It works great! The 036 I rebuilt with new cylinder kit reads about 155 psi. YEAH! ( instead of 90 psi with my old tester ) The tester was only $10. You have to twist the hose to thread the tester into the spark plug hole. When I did that, the fitting spun a little so I epoxied the fitting to the brass crimped fitting so it wouldn't turn. Then I wrapped black tape around the hose for the first 4 inches to take the stress off the hose. Should hold up now!!!!!
    Thanks for all the help! I love this site!
     
  13. atvdave

    atvdave ArboristSite Operative

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    Just had the same problem with my tester. I did a cold compression test on my Dolmar and got a 155 psi. I started it up to get it warm for a hot compression test and when I put the threaded hose back into the saw I twisted to hard and the fitting spun like yours. Now the tester will only go up to around 90 psi and start to bleed back down.
     
  14. wendell

    wendell Tree Freak

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    That's interesting. I have a HF tester and every saw I own is 120, even ones I know are pushing 200.
     
  15. hlhopp

    hlhopp ArboristSite Lurker

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    Thanks for the response wendell and atvdave! Seriously, I was beginning to think my saws all had 90 psi compression. I even put a different guage on my old compression tester and it said 90???? I would advise anyone with a compression tester like mine where you twist on the hose to thread it in, to glue the fitting to the crimped part. I first cut some grooves on the face of the fitting to give the epoxy something to hold onto. There is no way it can spin and break the seal now!
    Thanks again, gotta get out and have some fun makin wood chips!:msp_smile:
     
  16. wendell

    wendell Tree Freak

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    Can you post a pic of what you did?
     

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