Hey Gbauer, thanks for reposting the pictures. I didn't understand before, but now I see it clearly. Just to check, I went back and looked at the lifters on the spare engine and found one (cyl 1 intake) that had nearly half the brown marking of all the others.
Back on my original engine. Cylinder 5 intake there's a funny sheen that's part way up with the brown going low. It may be that there's an issue with the intake on the CAM which would indicate why it's getting worse, although I don't understand how that would make the exhaust valve run hot nor how it would create backfires.
It seems most likely to be a problem with the head and not the CAM. The perfect solution is to put in a more aggressive CAM.
The harmonic balancer is right on. I had to advance the timing to 20 BTDC to prevent constant backfiring through the carb.
Below are the two lifters on the left from cyl 5&6 intake. On the right are the first four lifters from my spare engine.
