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I was checking a no start situation on a Stihl chainsaw. I checked the compression, only had 60lbs. Not enough compression to run. Then the customer asked about the compression release. I pushed on the release still got 60lbs.

How does the release work? What trips it off? Shouldn't the compression be more than 60lbs to start the saw even with a compression release?

Sorry for so many questions, but I don't work on many saws with a compression release. (I do not remember the model number of the saw, and the customer took it home)

Thanks for your help.
 

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The compression release closes as soon as the engine starts from the expansion of the burned fuel when ignited. You can pull on the compression release button to manually close the valve and test the compression with the valve closed.

A compression release does not affect the crankcase pressure as it only releases pressure on the up stroke of the piston, so mixture intake is not affected. Low compression due to damage to the piston, cylinder or rings results in low crankcase pressure and vacuum as well as reduced compression.
 
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