there is no break in period, the jet is brass, solid brass, how does that "break in" and change mixture at any point? it doesnt. ball bearings are the most highly toleranced parts that are mass manufactured on earth, what exactly "breaks in" on them? plain bearings dont make metal to metal contact EVER, so how do they "break in"? the piston rings get one chance to conform themselves to the cylinder wall, and thats in the first five to ten minutes. that is the ONLY "break in"... which takes high pressure, meaning...full throttle, full load!(once theyve idled for a minute or two and are nice and WARM) five minutes....not really that long is it?
i avoid these "gentle break in" recommendations, and havent had any engine tell me afterwards that i did anything wrong...just the opposite, actually.
if it revs nicely but splutters or "fourstrokes" at full throttle, and returns to a smooth sound up hills, its too rich. if it revs out nice but tends to die at full throttle...
what you have is a slightly lean mixture, caused either by dirt in the fuel blocking the jet, some type of debris in there, it came with a(very unusual) small jet, or... you live at a ridiculous altitude!
check for dirt, or drill the jet out another 0.05mm...