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An engine is essentially a self-powered air pump, so it needs good compression to run efficiently, cleanly and to start easily.
As a rule, most engines should have no more than 10% difference between any of the cylinders.
Low compression in one cylinder usually indicates a bad exhaust valve.
Low compression in two adjacent cylinders typically means you have a bad head gasket.
Low compression in all cylinders would tell you the rings and cylinders are worn and the engine needs to be rebuilt.