RE: Is it a phenomenon of these little 2-strokes that until well broken in, they will not turn a larger
Don't know about the other engines. Running a .15 on a 10-4 is a totally unfair comparison, and I wouldn't pay any credence to the results myself. It is not uncommon for small engines like .15's to suffer with large loads like that, then wake up dramatically when you let them breathe. 9K to a .15 is IMHO like 5k to a .61.. or whatever, but I am just saying I don't think it tells you anything except how fast it manages to spin a 10-4 under protest. A .21 is over 33% more displacement, and they are apples and oranges.
MJD