Soot on leading edge of oil filter area

[clofan]

I've noticed some soot accumulation on my right-side engine casing. On the leading edge, and on top. See picture-- I had already wiped most of it off, but the orange outlined area is where it was. There is still some grease/grime around the bolt there, maybe that's where it's coming from? Although it was more like powdery soot, not anything real viscous. Is this common on these bikes? There's a guy in my area who also has a GZ250, and I happened to walk by his bike yesterday and he has the same soot accumulation there.

I plan to give the bike a real thorough cleaning this weekend, so hopefully I'll be able to determine the deposit's origin a little easier if it comes back.

E: I also plan to pull the spark plug and take a look this weekend, but I don't think it's a rich mixture issue. Could be, I suppose, but I get no smoke out tail pipe and last time I checked the plug it was a nice fine tan.