All the pistons should be at, or very near TDC. Wear in a cylinder happens mostly in the first inch of piston travel down the cylinder, so the tension on the rings will be at a minimum near the top of the stroke. DO NOT tinker with the pressure regulator while testing- - - -you're defeating the purpose of the test if you do that. Line pressure needs to be adjusted to exactly 100 PSI with no air flowing. Once the air line is connected to the cylinder, the pressure on the second gauge indicates the percentage of leakage. For instance- - - -100 PSI line pressure with no flow, and a cylinder pressure of 88 PSI=12% leakdown. There is no need to disable the valves- - - -just point the distributor rotor to the plug wire of the cylinder you're testing, which will assure both valves are closed. A leak rate of 10% or less is acceptable. Any more is an indication of leakage somewhere- - - - -air from the carburetor is a leaky intake valve- - - -listen at the tailpipe for exhaust valve leaks- - - -air from the oil filler indicates leaky rings. Bubbles in the radiator indicate a cracked head or head gasket problems. "All of the above" is why I rarely, if ever waste time on a compression test. Cylinder leakdown tells me where, and how much leakage there is.
Jerry
