The problem is, no two compression gauges are likely to show the same pressure, even when testing the same cylinder. 100+ PSI is probably good, as long as all cylinders are within 5%-10% of each other. A better test than compression is probably a cylinder leakdown test, where a known air pressure is introduced into each cylinder with the crankshaft locked, and each one is measured for pressure loss. For instance, set a pressure regulator to 100 PSI with the air line blocked, then plug the air line into a spark plug hole. Listen for air leaking from the carburetor, tailpipe, and oil filler. If the pressure drops to 90 PSI, that cylinder has 10% leakage, and you can tell whether rings or valves are causing the leak. Bubbles in the radiator indicate a head gasket leak. Check all the cylinders and compare results.
Jerry
