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What PSI is "good compression"

Discussion in 'Chainsaw' started by rwoods, Mar 13, 2010.

  1. rwoods

    rwoods Addicted to ArboristSite

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    I purchased a ProMac 8200 off eBay. It does not have a compression release and it cranks very easily. In fact you can turn the flywheel with your hand and you can barely feel the compression. On the other hand, I have a ProMac 700 and it is very difficult to pull the starter cord unless you use the decompression valve. You can turn the flywheel on the PM 700 with your hand but it will move the saw and it builds up a lot of resistance way before top dead center. It feels like it has more compression with the valve open than the PM 8200 has. However, using the same new chain on a new 28" sprocket bar in 36" red oak the PM 8200 outsaws the PM 700 maybe 20% faster (just made 2 cuts first one with the PM 8200 then one with the PM 700 - so the PM 8200 had the advantage of the freshest chain but that shouldn't be too much of an advantage; and yes the PM 700 seemed to be running right). I thought the 12cc difference would be effectively cancelled out by the huge compression difference. Long introduction to how much psi equals "good compression" as I see these terms tossed about on this site? If I can find a gauge to borrow, I'll measure the two. Replies appreciated.:newbie:
     
  2. rmh3481

    rmh3481 Addicted to ArboristSite

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    150 psi or better is considered 'good'. Of course the greater the psi, the bigger the pop up to a point. Maybe you need a set of piston rings? Have you removed the muffler and inspected the piston and cylinder?

    Bob
     
  3. rwoods

    rwoods Addicted to ArboristSite

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    Haven't removed muffler yet as it appears I have to dismantle the bottom frame to remove it. Here are the results of my compression tests (bearing in mind the guage is borrowed, is of unknown quality and probably not designed for low readings*):

    PM 8200 - a hair over 10 psi (yes, ten)
    PM 700 DSP open - 70 psi
    PM 700 DSP closed - 130 psi (it is very difficult to turn over with the recoil start; actually pulled it sideways and whacked my knee pretty good. I've own this saw since new - January 1982 - and it has always been this way.)

    How does the PM 8200 cut with such low compression? Would rings alone account for this? I thought about putting some motor oil in the cylinder and see if that made any difference - would such a test be worth the trouble in trying to decide whether or not to spend any more money on this saw?

    Thanks,

    Ron :confused: :help:

    * I checked the guage against two other air pressure dial guages; tied all three together to an air compressor. The test guage read about two pounds low until it stuck at 90 psi. I took the pressure up to 100 psi and it shook loose but was about 5 psi low.
     
  4. sloch24

    sloch24 AboristSite Guru

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    A saw won't run with 10 PSI compression... Check to make sure that your compression gauge has a schrader valve in the tip. A standard automotive compression tester will not give an accurate reading on a small engine.

    You're not getting accurate readings with that gauge.

    I'm sure others will chime in shortly.
     
  5. barton174

    barton174 ArboristSite Operative

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    How many cycles did you run it through? Probably take at least 10 to get the pressure in the line and gauge up enough to even out...

    ETA: It does have a check valve of some kind, right? So it holds pressure after it's unplugged or you stop cranking?
    Mike
     
    Last edited: Mar 14, 2010
  6. rwoods

    rwoods Addicted to ArboristSite

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    The guage does have a check valve so it will hold the reading. I'll take it to a saw shop this week and let them check the compression. Is there a rule of thumb as when you should scrap a chrome cylinder saw? Thanks, Ron
     
  7. thomas72

    thomas72 Addicted to ArboristSite

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    It really depends on the saw. The McCulloch sp125 is just about at its best at 175 psi / depending on which type combustion chamber you have. The Poulan 71A is just about at its best at 135psi and will still run on 90psi. Compression is relative to a lot of factors to that particular engine. I have a promac 800 and it has 180psi. I really do not care about compression too much if I feel the saw is making the power needed to get the job done.
     
  8. rwoods

    rwoods Addicted to ArboristSite

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    I have been led to believe that the PM 800 and the PM 8200 are essentially the same except for a three piece crankshaft in the PM 8200. So I expected 175 to 180 psi. It seems in the Mac literature some 800's had DSP and some didn't; this is also consistence with what I have encounted while looking for a "new" saw. Since my PM700 is almost impossible to hold down and crank without opening the DSP, I kept wondering during my search for a more powerful MAC how in the world was I going to crank an 800 even given the foot pad. I have even checked out the parts manual for a hidden automatic compression release but I don't see one (nor have I ever heard of one on a MAC).

    I bought this rough looking PM 8200 after just missing some really good looking saws in the last two months: a 800, a 805 and a 850 - I guess I am just too cheap.

    Any advice or sources for a better MAC would be appreciated. I'm not interested in a MAC without a chain brake. I still have by grandfather's old gear driven MAC bow saw which send me to the hospital 30 years ago as a result of me showing off with it and I haven't run it since. It could have decapitated me but I hung on to it and it went over my head. The efficiency of a bow still tempts me, but I just look at the old saw and I get over it.

    Ron
     
  9. thomas72

    thomas72 Addicted to ArboristSite

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    All I can say is look at your local saw shops. I found my 800 at a saw shop 4 miles from my house and paid $10.00 for it. I am like you, I do not spend a lot of money on something unless I can get a worth while profit off of it.
     
  10. rwoods

    rwoods Addicted to ArboristSite

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    Okay, I put the saw's recoil starter back on the PM 8200 and gave it about 15 pulls in a row - gauge reads 131 psi. I repeated this several times with same results. So I hooked the PM 700 to the gauge and tried several times to replicate but couldn't get a good rhythm going as it is too hard to hold down and pull; nevertheless I got my previous readings of 130 psi every time. Since they both read the same, that would explain the performance differential. But what does it mean, if any thing, that I have to build compression with the PM 8200 while the PM 700 appears to go to full compression immediately? Should I be looking at a rebuild, a new saw or just finish replacing the missing pieces and use it as is? Thanks, Ron
     
  11. Diesel Pro

    Diesel Pro AboristSite Guru

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    Gauges will vary based on the style of the end, etc. I consider my Snap On Motorcycle compression gauge kit to be of very high quality yet it reads lower than some I have seen here.

    All cold with throttle wide open:

    372XP (new just 2 tanks through) 145 psi
    359 2002 model low hours excellent condition 145 psi
    Echo 351 moderate hours excellent condition 145 psi
    Homelite Super 2 moderate hours runs quite nice (low end quality saw) 125 psi


    I've seen a 359 with much higher psi and feel that it is due to an "easy" gauge or difference in the adapter. I have a Milton gauge as well and plan to try it soon for comparison sake.
     
  12. rwoods

    rwoods Addicted to ArboristSite

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    Does this mean I need to test it again with the throttle open?

    I have run saws for years but other than bolt-on stuff I have always relied on saw shops. No one wants to work on my Macs any more (they are older than most of the mechanics) so I am trying to learn to fix them myself and thought what the heck if I have to do it why not buy an old saw and fix it - thus the PM 8200 (not really an old saw as it will only be 18 this year). But as you can see I have gotten over by head real quick. Help and advice is welcome as I am still wondering whether I should just look for another saw instead of spending a lot of time and money rounding up parts for an obsolete saw that may be worn out. I love the sound of Mac's, the fond memories of my dad's first big Mac that he used to cut pulp wood, the American innovation, the looks you get from other cutters when you fire one up and let her rip, etc etc. but warm and fuzzy emotions* don't get the job done. Ron

    * Mac's that won't start also stir some other pretty strong emotions for me, as well as they did for my Dad (died at age 81), who switched to Husky's in his later years. I have switched for light work to an 036 Pro and can't bring myself to spring for a big new Stihl for felling and bucking the bigger stuff. Given the abundance of replacement plastic parts that can make a saw look new, I am leery of buying a used modern saw as I don't know enough to know whether or not the mechanicals are any good as evidenced by my questions here.
     
  13. Diesel Pro

    Diesel Pro AboristSite Guru

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    I don't think it is necessary to open the, just an old rule of thumb. Should get your peak number more quickly.

    A better test for ring seal would be a cylinder leakdown, but you'd have to lock the crank from turning.


    FYI My Harley is 190-195 psi if memory serves reduced from 205-210 by running larger cams to tame it down.

    Most of my experience is 4 stroke stuff...
     
  14. rwoods

    rwoods Addicted to ArboristSite

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    Thanks. I may rig some thing up next week and try a leak down test.

    To me Macs are to chainsaws as Harleys are to motorcycles as radial engines are to airplanes - nothing else sounds right. Too bad the employees didn't buy McCulloch.

    Ron
     
  15. barton174

    barton174 ArboristSite Operative

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    The pressure should be higher with an open throttle... or at the very least, go up faster... Should be higher, though...

    Mike
     
  16. nmurph

    nmurph ArboristSite.com Sponsor

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    is the valve at the tip of the hose?

    i have a near-new PM700 and it only registers 130.

    opening the throttle will not have any bearing on compression. it might make it rise quicker, as some has suggested.
     
  17. J.W Younger

    J.W Younger Tree Freak

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    In my experience the throttle has no affect and if you look at it the exh port is wide open to atmosperic pressure .
    I quess it dont hurt to set the fast idle to make sure theres no vacuum in the crankcase pulling the air out before its compressed.
     
  18. barton174

    barton174 ArboristSite Operative

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    Fair 'nuff... I know it does matter on motorcycles (L4, large displacement), but then, the intake tract is smaller on a bike, compared to the piston size, anyway...

    Mike
     
  19. Saw Dr.

    Saw Dr. Junk Collector

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    If your 8200 runs, cuts, and idles like it is supposed to, what is the problem? I would not tear into that thing if it is working properly. There are several parts on those that are not easy to find. The piston is the hardest to come by, and it is not quite as easy to change as others, due to having to press the needle bearings into the piston. It would be very easy to create an air leak on that one since it is a clamshell engine. Air leak = junk unobtanium piston. I would just run it and have fun if it were me.
     
  20. rwoods

    rwoods Addicted to ArboristSite

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    Valve is at the base of the guage.
     

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