My engine surges when rich - but it didn't do it before getting a new ring. I broke it in on an old plug and put a new one in it before I did a peak-out run on it. I'll be trying different props, fuel, and another plug to see what happens. My engine surges when rich and it will go rich of lean with the needle.
I think a surging engine that doesn't go rich or lean with needle adjustments might be getting starved for fuel. Doesn't hurt to try a new plug first. I've never had a glow plug engine surge before, so this is "weirdville" for me.