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Chainsaw Compression Question

Discussion in 'Chainsaw' started by Spoon Carving With Tom, Sep 18, 2019.

  1. pioneerguy600

    pioneerguy600 Lost in Space Staff Member

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    Nope, doesn`t matter, open or closed one will still reach maximum compression if engine is turned over enough times.
     
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  2. Ax-man

    Ax-man Addicted to ArboristSite

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    If this turns into some kind of war of opinions I am on your side.
     
  3. Duce

    Duce Addicted to ArboristSite

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    How does that happen? Gauge will only maintain and read highest compression pumped into it, or does it measure something else?
     
  4. pioneerguy600

    pioneerguy600 Lost in Space Staff Member

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    Thanks, may need help.:)
     
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  5. HarleyT

    HarleyT Tree Freak

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    What happened to the post you quoted?
     
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  6. HarleyT

    HarleyT Tree Freak

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    What about the location of the Schrader valve?
     
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  7. Ax-man

    Ax-man Addicted to ArboristSite

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    Duce, I see you got ahold of my post after I edited it to save some controversial arguments. Primary compression is the condition of the seals . When you do a pressure /vac test on the bottom end you are measuring in a way your primary compression to see the integrity of the seals . Secondary compression is at the top end of the stroke which is measured with a regular compression gauge.

    Have you ever seen any published compression numbers from Stihl??? You won't because they consider primary compression more important. I know it is confusing but it is what it is. I have been down this road before and kind of started a fire but I thought I would put it out before it spread.
     
  8. HarleyT

    HarleyT Tree Freak

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    That's no fun!!!
     
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  9. Duce

    Duce Addicted to ArboristSite

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    You still only get a highest reading on any gauge. Did not say it would be a good reading with a large volume adapter either, or location of valves. :crazy2:
     
  10. HarleyT

    HarleyT Tree Freak

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    Nah, we should get some action over the schrader valve location!!!
     
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  11. Spoon Carving With Tom

    Spoon Carving With Tom ArboristSite Operative

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    It’s not in the screw that locates in the spark plug hole or start of the tube that’s for sure!
     
  12. Huskybill

    Huskybill Addicted to ArboristSite

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    I just looked at you tube. One guy, I gather he has a repair business, he does chainsaw compression tests both ways.

    I guess no matter which way we stir the pot everyone is correct. Some stir the stew counter clockwise, some stir clockwise it tastes the same.
     
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  13. Duce

    Duce Addicted to ArboristSite

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    What?
     
  14. Huskybill

    Huskybill Addicted to ArboristSite

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    What? What.?

    I had a procedure today I’m a tad slow not enough sleep
     
  15. TheTone

    TheTone ArboristSite Guru

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    I'll jump in on the Schrader valve question. Two things you have to have to get an accurate compression reading on a small cylinder (such as a chainsaw).
    (1) The right valve - one with a low opening pressure, about 1 psi, not one from a tire, and (2) the right location - at the spark plug attachment, not at the gauge end of the hose. Basic physics: PV=nRT, the ideal gas law. When the valve is at the end of the hose, you have added the volume of the hose to the compression volume. No matter how many times you pull, compression will level off at a lower value than if the valve is at the plug. Why do gauges generally have the valve at the end of the hose? (1) Convenience, the pressure release valve, (2) the gauges are usually used on cars - how much difference will the added hose volume change the compression reading on a 427 cu.in. V8? Not much; on a chainsaw, enough to throw the reading way off.
     
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  16. hotshot

    hotshot ArboristSite Operative

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    Seriously? You simply leave the test blind on!

    I once did a compression test right after I vac & pressure tested an 056 Magnum that I just rebuilt.

    Scratched my head for a few minutes, as I was way short of the expected 150 psig.

    If you’re totally blinded like that, it’s makes a huge difference on a 2 stroke engine, because it’s starved for atmospheric air.

    The very first pull is as high as you’ll get on the gauge.
     
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  17. Duce

    Duce Addicted to ArboristSite

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    Have not done that yet, but give me time. Now, I understand how it could be done.
     
  18. Bob Hedgecutter

    Bob Hedgecutter ArboristSite Operative

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    Tom, when your brand new saw only gave a 25 psi reading- was the decomp valve depressed?
    Cheaper gauges can be used- may not give a true reading, but if you set them up on the powerhead correctly and pull a few times until the needle will not go any higher on a known good saw like your new one- usually you can get a benchmark for that gauge. Equally if you know you have a saw not making compression, you can get a no go benchmark pressure for the same gauge.
    But 25 psi on a new saw, I suspect something was not sealing well- or a goofed from the factory gauge.
     
  19. Spoon Carving With Tom

    Spoon Carving With Tom ArboristSite Operative

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    Hey Bob, na not compressed. Took the thug apart to check for leaks but everything seems fine. Looked at reviews online seems like more duds than successes for that model. I have gotten a new one from All Mower Spares. Hopefully I have better luck!! :)
     
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  20. Bob Hedgecutter

    Bob Hedgecutter ArboristSite Operative

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    Supercheap Auto mate....... super cheap is not always super good.
    Be interesting to see what kind of reading the new gauge gives you on the new saw.
     

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