Compression test what's good and what's bad

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Many saws will start and run well at 100 psi but without a lot of power, if it gets as low as 80 psi it probably won't even start. It all depends on what you want to do with the saw. If it has low compression but it's just a saw for occasional use, just run it till it won't start. If it's a saw that needs to be dependable, will see heavy use and needs to have original power then it's time for a top end rebuild.
 
Many saws will start and run well at 100 psi but without a lot of power, if it gets as low as 80 psi it probably won't even start. It all depends on what you want to do with the saw. If it has low compression but it's just a saw for occasional use, just run it till it won't start. If it's a saw that needs to be dependable, will see heavy use and needs to have original power then it's time for a top end rebuild.
Yep, nothing worse than getting a saw running well at the shop, then going out on a job just to have it fall on its face in the woods! Put that Saw in the hands of a guy you know that has to depend on it for week or so, he’ll tell you wether you have a winner or not!:cool:
 
From what I have learned here, the compression testers themselves can often give false readings on chainsaws for various reasons.

Usually due to (1) the placement of the Schrader valve at the end of the tube away from the cylinder, (2) a tire type Schrader valve instead of a low pressure opening valve. There are many threads on this forum about this. Do a search for, I'd start with, "Schrader valve" and you'll find more than you ever wanted to know..
 
There are other variables in play. A good running Husqvarna 350 will blow 120. I tested an 034 tonight that hasn't ran in some time and was visibly dry through the exhaust port.
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Get the fuel system in order this one will probably be over 150 as good mix loosens everything up. That said, on my bench I'll usually go in it and clean everything and check ring end gap before I try and get much more than a pop to test ignition.
 
Modern piston ported saws generally need to be over 100 PSI to start and run. Older reed valve saws may still run and cut well even down to 75 or 80 PSI.

Certain saws like older (real) Jonsereds models had exceptionally high compression, 180 to 200 PSI was not at all uncommon even on saws with lots of hours.

The Schrader valve needs to be in the end of the hose with the spark plug adapter to minimize the volume "above" the piston to achieve a more accurate reading. In addition, the Schrader valve needs to be one intended for a compression tester as they have a much lower pop off pressure. I made a side by side test with the low pressure Schrader valve from Snap On intended for a compression tester and one from a tire, there was 10 to 12 PSI difference in the compression readings based on which valve was in place.

Mark
 
If a saw has been sitting a long time what's the best way to go at getting a good reading. If you add oil it will be high. But if its dry I could be low
 
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