I've seen good-running engines with compression pressure as low as 60 PSI. The trick is to have as little variation between cylinders as possible so there's very little difference in power impulses. One cylinder that's very low on compression causes a lope as the engine runs.
Another diagnostic tool is called a "power balance test". Connect a vacuum gauge and a tachometer and run the engine at a constant fast idle. Disable the spark to a particular cylinder by pulling a plug wire and watch the drop in RPM and manifold vacuum. Disabling a cylinder that's not pulling its weight will result in little or no change in RPM and vacuum, and one with a burned intake valve might actually cause the engine to speed up due to less dilution of the intake mixture. An even more definitive version of the power balance test is done by running the engine on only the companion cylinders (1 and 6, 2 and 5, and 3 and 4, for instance) and monitoring RPM and vacuum.
Jerry
