What PSI is "good compression"

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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.

I appreciate your comments. I didn't know what a clamshell engine was until today. I'll be careful if I tear it down to replace the rings. The reason I bought this size of a saw is to fell large oaks - I get my firewood from dying oaks that are usually 24" or greater at the base. I usually use a ProMac 700 with a 28" bar. Occasionally I wish I had a saw with more power. Once the tree is down, I usually buck it with a Stihl 036 Pro with a 25" bar. I also always have a back up saw when felling large trees. I don't like walking behind a tree as I fell it; sometimes the Stihl is too short (and not strong enough to pull the next size bar). I have always been led to believe that the 5.0 cubic inch Mac's were hotrods compared to the 4.3 cubic inch Mac's. I just want to get the potential out of the 8200 and since the post office lost a few critical pieces I didn't want to spend money replacing everything just to have a mediocre old saw.
 
IMO
130psi - poor
150psi - pretty good
175psi = new saw or good old saw
175psi+ - has someone removed the base gasket:)

I ALWAYS look for 150psi + on all saws and will re-ring/new piston at 130psi after inspection shows wear or thin rings from wear.
Spud
 
My Husqvarna 268 is 160psi - Has plenty of power

My Poulan 3400 is 90psi (3rd pull) 105psi (5th pull) - No power at all. After 5 min of running it can barely cut through a 8 inch log

My Poulan 3700 is 130psi (3rd pull) 150psi (5th pull) - Umm, not running yet so I don't know! But sounds like its in good shape.
 
B200's advice is good. If your saw will pull a 3' bar like it is why take it apart. Parts are just getting harder and harder to find for those old saws. The saw will either cut what you want or not. If it gets the job done what else can you ask of it. If it were my saw I would run it like it is and on the side start buying replacement parts. Then if it wears out to the point it will need to be rebuilt all the parts will be on hand and ready to use. More than likely unless the saw suffers a catastrophic failure it will probably never wear out under your use.
 
Compression is a relative thing. I have run old 044/046s that would fall like a brick when you do the starter rope test on them but still cut good enough for me to produce. I have a sp81 that had killer compression until not long ago, now I can hear the fuel bubbling past the rings and compression feels lower. That really pisses me off, but you would never notice while running the saw...
 
Thank you to you who encouraged me to leave well enough alone. But before I took this advice, I took the PM 8200 to a saw shop. The sales lady said the mechanic said the compression was good. I ask how much did it have. She inquired and relayed back that it had 120 psi; and they don’t work on Mac’s (afraid they might break something that can’t be replaced). The other mechanic happened to be walking by, overheard the psi statement and said to me, “120 psi is not good, a saw needs to be rebuilt at 130 psi.” Took the saw home and checked it and my PM 700 again – read 131 psi and 130 psi respectively just as before. Out of curiosity, I checked my Stihl 036 Pro – 120 psi. Checked it several more times 120 psi every time. My Stihl is just like the PM 700 – very difficult to turn over with the decompression valve closed. So I decide to forget this chapter of my life and get on with repairing the PM 8200 and leave the internals alone. To the credit of the USPS, they quickly and without any hassle paid my claim for the lost chain brake/side cover, the bar and chain (I forgot to include the dogs and I didn’t ask for anything for the muffler since it was cracked already). I recovered enough to buy a new bar and chain, new dogs and a used chain brake/side cover off a PM 850 (couldn’t find one for a PM 8200 nor a PM 805 which appears to be the same). I have more to do to it: bar bolts are stripped, muffler needs to be welded back together and a little paint. Below are some pictures of what I bought, what I got after the package was destroyed in transit, and a picture of it today when I was using it to fell a crooked white oak. I don’t know how it is supposed to run but I put it in a section that was a tad over 20” and it cut straight through it as if I were cutting limbs (no engine bogging and no rocking of the saw) and that’s good enough for me.
Ron Woods :clap:
 
In 20 years I have never used a compression gauge on a two-stroke single. It can tell you if it is bad but not if it's good. I have seen good readings from a cold saw that has sat for awhile. The fuel mix evaporated from the cylinder wall and the film of oil is left behind. Acts like new rings. On multi-cylinder two-strokes it is excellent as a comparator between cylinders. On four-strokes it is a useful tool, but not as effective as a leak-down tester. On a chainsaw, a visual inspection of the piston/cylinder is far more informative. If you poke a tiny led light through the spark-plug hole you can see the exhaust side. If you are ever suspicious, remove the muffler and look inside. Cheers, Simon.
 
Yep as 056 said "compression is a relative thing" especially on saws. It will vary greatly from saw to saw depending on the design and build. Single ring, two ring, domed piston, flat top or dished etc. All of these will affect what you get for readings. Remember you are testing at a piston speed of what?, maybe 100 RPM by pulling the cord. And it idles at 2700 RPM and revs out at 10,000. Also what may feel like wimpy or really strong comp on the cord will have a lot to do with the starter pully size or even the size of the dia. of the cord itself. A friend of mine has a Dolmar 133 that is the GDest arm breaker/finger tearer I've ever started but the most we can get on a gage is 175 PSI. A lot of it was in the starter. It had a small dia cord so you were pulling from a leverage point smaller and closer to the crank. I replaced the small dia cord with one that would that would just fit in the pulley and completely filled the the pulley when recoiled. You still have to treat that saw with respect when starting but it pulls quite a bit easier now. I agree with the general theory "if it ain't broke---don't fix it" especially on old saws that parts are difficult or impossible to find.:cheers:
 
does compression matter in a 2 stroke?

one thing that occurs in a two stroke piston ported motor/engine is that piston speed alters the compression ,second is there are pipes on some that leak down some air .third factor that comes to mind is the case if leaking somewhere like end seals , alters the cranking speed test & affects the breathing +/or squish at running speeds. Fourth . Lastly,if the rings cause an effective seal power can be great in a saw that slowly sinks to the floor when suspended by the start chord. :greenchainsaw:SO, as Simoniser says check the cylinder through the plug .yes indeed! Its the exhuast side that shows wear first & worst.Check the piston through the exhuast port . One is not always the same as the other(pist.cyl.).:chainsaw:Although they may make the right song it can be a different story in the wood where yu dont want it to go wrong.New second hand ? Shove it toward its task & then you get an idea?AND STILL after half an hour of fair performance some second hand saws I had would show a breakdown & start to give trouble only to run well when next started cold. :dizzy:And ported saws I have heard two ported supposedly the exact same way..one that goes like the clappers +the other ruined by the effort? IF IT AINT BROKE DON"T FIX IT!
 
This compression issue is hard to figure , I thought it would be cut and dry . The 359 that I purchased recently used and hard to start is running fine now , fixed the boot , no air leaks and its at 110 lbs.starts easy every time its a 2003 , for comparison my like new 346 NE 2009 is at 120 lbs. , My 25- 30 year old homelites are at 130 and 150 lbs . Maybe like many of you have said its not a perfect situation where we can come up with a rock solid figure . But how can a 30 year old homelite have 30 lbs more compression than a brand new husky ?

I guess what I'm after is a figure that I could by when buying used saws that would tell me if the saw is going to need rings and maybe a piston or if the cyl. is scored and that will need replaced also .
 
Gauge is only a year old , wouldn;t think its giving me a false reading , the old homelites read 130 and 150 , I'll try to get another gauge and compare them .
 
The only thing I can see after checking it out again is I'm not getting a prefect seal at the threads , the fitting on the gauge looks like its tapered , and it does snug up a bit but if I spray some soapy water on the threads I can see it suck the water right into the cyl. , I never really worried about an air tight seal when checking compression , could this make a big difference , I can remember some gauges having a o ring to help seal but this gauge doesn't and its a good brand name gauge.
 

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