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Discussion in 'Support & Announcements' started by Darin, Aug 30, 2015.

  1. jlfraf

    jlfraf New Member

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    I have a very old Stihl 08S chainsaw that needs the bar, chain, and fuel cap replaced. It has a I.D. plate on the top of the oil tank # 4816750, which I believe is the serial number. The original 17" solid bar is still installed, and it has 60 link 0.404 pitch chain. It was working until recently, and I would like to restore it. I have no idea how old this chainsaw is other than I inherited it from my step father years ago, and I personally used it when I was a teenager. Any help would be greatly appreciated.
     
  2. darryl siemer

    darryl siemer New Member

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    How does a newbie start a new thread? I'd like to add my two cent's worth to Arboristsite's discussions of what "compression test" results should be but all of its compression related threads seem to be pretty stale (>2 years old).

    Here's what I'd like to say:

    What we should expect to see when doing compression tests on small engines seems to be a perennial question. That question has also been answered wrongly far more often than correctly resulting in the waste of a great deal of time/money trying to fix "problems" that don’t exist.



    For instance, the relationship/equation often promulgated to calculate that figure ...


    C (compression) = Patmospheric * CR (compression ratio) ^ϒ where ϒ=Cp/Cv = 1.4


    ...is relevant only for adiabatic systems; i.e., ones in which the air’s compression heat isn’t lost. During a cold crank compression test, that air (a few cc or milligrams) is pumped through a relatively massive hose and/or metallic tube plus some fittings into an also relatively massive metallic (usually brass) bourdon tube-type pressure sensor. Since the gauge’s heat capacity is far greater than that of the air, the latter will be at near room temperature (have lost its compression heat) when the dial is read.

    This means that Boyle’s law (gas pressure inversely proportional to volume) is the relevant relationship. The bottom line is that an engine’s absolute “compression” (pressure) test result should be Patm x compression ratio (CR), not Patm x CR^1.4 . The irrelevant exponent (1.4 - the ratio of heat capacity at fixed pressure to that at fixed volume) biases the expected figure upwards by well over 100% meaning that 100% of the engines so-tested/judged would "fail" .



    Since relevant CRs (“corrected” or “Japanese” CRs - total volume above the exhaust port/(combustion chamber volume ) of two cycle engines typically range from 6.5 to 7.5, the highest cold cranking compression test result we should expect for chainsaw-type engines would be 6.5 to 7.5 times local atmospheric pressure (i.e., 95 to 110 psi at sea level – lower at higher altitudes).



    The relatively large “dead volumes” of automotive-type compression test equipment will also lower test outcomes. Dead volume is any additional space between the bottom of the spark plug’s hole and the gauge’s Shrader valve which serves to lower the engine’s CR during the test. With real gauges, it's typically on the order of one cc which figure represents about 40% of a small chain saw engine’s compression chamber volume which would therefore lower the test's result by roughly 30%.


    Consequently, I don’t expect and have never seen a compression reading exceeding 100 psi with any small two cycle engine, new or old.
     
    MountainHigh likes this.
  3. Mal

    Mal New Member

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    Hi there I have a pioneer farmsaw the problem is it doesn't seem to pump fuel . I've renewed all parts in the carb and used an external fuel supply .I fitted a prime bubble and if I keep pressing it the saw runs I've tried fitting a tube to the port that allows crankcase (pressure depression )after removing the carb , it seems to draw fuel. I'm at a loss as to what the trouble .is
     
  4. MountainHigh

    MountainHigh 45cc and 60cc

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    Yes, I am struggling to find the answer to that as well - you used to be able to just go into the chainsaw forum and post a new thread with your question/thoughts - (now it seems you can't post unless it has been a "sticky" (scratching my head) - and you can't post without a "prefix" to buy or sell or .... ) ! ?!?

    ** UPdate - the glitch seems to have passed and posts are now being accepted as usual. Not sure what that was about.
     

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