direct vs differential compression check and a valve job

[Dorsal]

I have only done direct compression checks on my engine and they were always on the high side (160-200 psi). I took the plane to a IRT @ 50 hrs to look it over and asked for a diff check but he didn't want to as it was too "involved"

-8 years later, 512 hours on the engine-
Lately I have been having a few concerns with the engine performance, too many blades to burp the engine, slightly low static run-up (50-100 rpm), subtle vibrations at full power, max to min direct compression nearing the 29 psi limit (#1 @ 190 vs #2 @ 160).

I took the plane to another IRT (great guy) who did the differential test. Turns out the test is easy (as I am sure many of you already know) in fact now that I have my own tester I find it easier than direct. That said the results were not good, 74, 50, 68 and 58 over 80

I pulled all the heads off, cleaned them and lapped the valves which looked a bit pitted. I replaced the intake valve on #4 as it looked like a re-grind was used when built

I now get a static run-up ~100 rpm higher than I have ever seen and the diff-comp numbers are much better (79,78,79,76 / 80). I have only flown for an hour but it feels smoother and now "burps" quicker.

This engine sat for ~7 years before being installed and run, I wonder if the valves and seats suffered some corrosion during that time.

In conclusion, if you don't already have one, get a diff-comp tester ($100) they are easy to use and, in this case, highly informative.