Compression test can be a tool to fine tune valve lash.
Set lash to: Intake @ 0.010”and Exhaust @ 0.012” using previously described method hot and running. Drive the car for several hundred miles varying rpm as if you were braking in a new engine. Hopefully this will free up any stuck rings, and help reseat rings if car has been sitting for a stretch of time. Then perform a second compression test, and observe any pressure differences from first test.
If you find that any cylinder pressures are 10% or more lower than adjacent cylinders, loosen that exhaust valve in 0.002” increments ( 0.014”) retest that cylinder to see if pressure increased. Engine will be a bit more clattery, but should run smoother at idle. What you are doing is making exhaust valve open later and close sooner allowing pressure to increase in that cylinder, in other words decreasing valve event overlap.
Camshaft lobe wear, valve recession and other variables in a worn engine will cause pressure differences as will ring and valve problems. My 225 has a big mystery cam with low vacuum at in gear idle because of a lot of overlap and with head mods making 9.5:1 cr. Engine always had uneven exhaust pulse when listening at rear bumper, and causing me to think there was a ring problem in one hole.
I have had to experiment to find an ideal lash setting, and discovered setting lash by compression readings, all cylinders were made the same within 2 psi at 163 to 165 psi. This smoothed out uneven exhaust pulse, increased & smoothed out in gear idle vacuum from a wagging needle between 1”- 6” Hg to a just a 2” sweep between 4”- 6” Hg. which 390 Holley 4v likes a lot better. I ended up with same setting for intake and exhaust, some cylinders at 0.024” ranging up to 0.028” on one cylinder that was at 140 psi.
