Buck-set mine at 30-31 degrees and seems to run fine, best i can get out of it is 7300 on 16x6 APC. Rings are in order here. I'm going to bring the timing back to 27 and see what happens. If i loose power to a great degree, I'll move it back.
I'm just finishing a 26cc Stihl, dual ring, 4 bolt cylinder. This thing is lighter than the Echo 21cc at 2lb 2oz. Have a small issue with carb spacer-need to fabricate a new one or modify the existing. I'll post a pix when done.