Flight Risk:
I thought I might be over-propping it as well, but with the partial choke it would run wide open really well with no sagging or surging. I am going to try an 18 inch prop just to see what happens, though.

Out of curiousity, I checked the sears website for parts and the carb for the 34cc and for the 40cc are the exact same part number. My ryobi 26cc also has what looks like the same carb.

Next I think I am going to put the 34cc on the stand and see how that does.

Again, I am not an expert in this field so if you see any flaws in my reasoning, don't be shy saying so!