how much compression is good

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120 psi is just a average base line for ok health saw

at one time i was told if it didnt pull 100 psi the rings were shot ,, now take into cosideration of the size of design of the motor, some pro saws have more than 150 psi. when i got my 362xp it only pulled 135 ive seen 25cc trimmer's barely pull 98 psi new, a trick for ya is too pull till it stops reading then pull it one time and see if the one pull is more then half of the total psi from the first reading , its a easy way to till if the rings are worn
 
Generally in a 2-stroke if the compression is falling under 100psi,something is worn,it's sick. Much less compression than 100 psi and it may be hard to start and lack power. Yet I've seen a few saws that only had a 100 psi new,and they ran ok.
A good range for compression on a saw is between 130psi and 160 psi,but there are always saws that stray from the rule.
A racing saw can have anywhere from 130 psi to 200 psi(Only in a few cases.)
High compression helps torque but can also fight high rpms. Some very high output engines must have a lower compression of about 135 psi or risk seizure and detonation problems.
You can usually go about 20 psi over your engine's stock compression without problems.
The old trick to raise compression in a saw is to take the cyl. gasket out or put a thinner one in,this will give more compression,but also lowers the exhaust port and transfer ports and this hurts top end power a little,but generally the comp. boost more than makes up for it...
 
Does this rule apply to a pioneer made in the 60's/70's?

Should a saw that was made 50 or 60 years ago still be compared to today's standards?
 
Yes,I'd say your Pioneer could run in the 130-150 psi range,I'm not sure(not having a cyl. in front of me) but I doubt your Pioneer has a hemispherical type combustion chamber with a modern squish band,(Like today's Husky's and Stihl's and other modern 2-strokes)so I think about 150 psi would be safe.
As for the really old saws the combustion chamber designs could be pretty primitive,and some also(Like my old 275 Canadien) ran more timing advance than modern saws,so I wouldn't get carried away raising compression much above stock,I think 130-140psi would be ok.
 
It should have about 90-100 psi to run. It probably has worn rings and/or piston damage.
You can remove the muffler and peek at the piston,look for any seizure marks on it,broken rings or stuck rings-you should be able to push on a piston ring with a small tool and it should move slightly,spring in and out.
You will probably have to remove the cylinder,light damage on the piston and cylinder can be repaired easily,if the rings are stuck use absolute care to free them without breaking them(I'm assuming your saw may be too old to get new parts),for stuck rings you can try soaking the piston in carb cleaner or engine parts cleaner.If you can find new,or good used rings great,put them in.The cylinder can be best finished up with a ball hone.
If you can get replacement rings and the ring grooves in the piston are carboned,snap one of the old rings in half cleanly,and use it as a tool to scrape the rings grooves clean.Use care and precision doing this,don't scratch or gouge the ring grooves.:msp_wink:
 
I agree with trapper! I've seen them run on 60psi, I've no idea how though! General rule of thumb is 90psi or higher for a two stroke, 50psi or higher for a four stroke.
Little antidote, I had a buddy call me a while back, one of the best car mechanics I know, really knows his stuff! He was working on a lawn mower with a 6.75hp Kawi on it. He asks me how much compression it needs, he can get it to run, he says he can prime it and it runs, but not well.
I tell him what I stated above, he responds, "OH! That's not good, I've only got 35psi out of it!":laugh: He pulled the head and the head gasket was blown. Odd and rare for these engines, but this thing had been abused, so nothing was surprising.
I just found it funny that someone who knows so much about cars couldn't relate the same knowledge to a small engine. Same thing, same rules, just smaller.
 
my 2 cents...

my experience has shown anything less than 100 on a two stroke isnt going to fire. had one saw with 95 and it would puff smoke when trying to start but couldn't fire up. most saws with 120 would run decent enough to cut while my older(60's) mccullochs and homelites had 120 and would pop quite well and feel very strong. i think their engine designs with larger bores and shorter strokes made their compression "measure"low. now my newer makita 6401=180psi. and my husky twins (1100CD and 2100CD) 145 and 155psi and without decomps with flat break a d handle in half ifn your not careful.
 
Rule of thumb...

The three pull thing doesn't cut it on many of my saws. I have multiple older saws and some newer ones that pull around 120# to 135# after 6+pulls. They all start cold on the 3rd-5th pull and idle/cut perfectly all day long.
 
I tested a couple of 3400 Poulans and compression was about 100 psi stock,surprised me but we know they run ok. Yet I've seen motors that won't run with less,the difference I found in some cases was if the piston had seized or damaged rings but holding 90-100 psi,but when it fires,combustion blasts past the piston into the crankcase and they won't start.
 
Yes I'm not a fan of really high compression in race or modded work saws... but if you are building smaller saws (less than 70 cc) comp. of 170-180 will definitely give it more torque and pulling power. In smaller saws you just can't put too much port duration in them or they can lose most of their torque,and will constantly bog in a cut and be almost useless. On these the compression boost helps quite a bit. In the old saws(60's etc.) increasing compression to 150-160 will help too,but because those old motors used a usually a poor combustion chamber design, detonation and pre-ignition is a risk causing piston seizure problems if you go higher.
 
Elevation is an important factor when reading compression. I live at 9400’ and the highest compression reading I have seen on any of my saws up here is 148psi. The same saw blows 170psi at 5400’ and 200psi in Georgia at roughly 800’.

Also, lots of compression guages read low. In general I find compression to be a misleading tool when diagnosing a saw. Looking at the piston and a good old pressure/vac test tell me a lot more.

When I do a compression test I pull until the needle won’t rise anymore. Often 10 or more pulls. It’s a workout up here. Maybe that’s why I don’t really like compression tests.
 
When i went to Yamaha school they told us there is many poor inaccurate comp. testers,said in their testing the Snap-on was the best,that's what I have,I doubt an E-Bay tester is as good.
Yes high altitude lowers comp. and also leans the fuel/air mixture. So more comp. is good,but I think a carb "bore job" would help to get more of that thin air in.
 
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