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PSI will depend on the engine and some on elevation. Compression in an engine that is running normally can be from about 120 to 150. Engines can run fairly well down to 100 psi, lower than that, there is usually a notable lack of power and often hard starting. If you have less than 80 psi, it's almost impossible to get 'em to run.
 

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Getting the correct reading in the first place is the hard part. You'll get all kinds of variations on a theme, but I generally do it like this:

- Both plugs out, motor cold
- Connect gauge to each cylinder in turn
- Hold throttle wide open and pull 3-5 times HARD

By the time you hit your third, maybe fourth pull, the gauge should no longer be climbing. Take your reading. When you're done, compare the readings to each other. They should be within a few percentage points of each other. A significant difference, even if each would individually be considered good, is indicative of a problem.

120+ generally is "all good".
110-120 is generally fine even though it's showing some wear
100-110 might run ok, plan for a new top end off season
<100 and you should do a new top end as soon as you're able

Low compression will cause hard starting, poor performance, poor mileage, premature plug wear, etc.
 
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