If you're using the stock muffler, you don't need more back pressure. Are you using a 10 x 6 prop, if you have too big of a prop it would act the way you describe. Your tank center line should be about 3/8 inch above the needle valve, and the tank should be 10~14 oz, (Sorry I don't think in metric). You never said if it was broken in, is it? If you see black in the oil residue, thats aluminum wearing, that's bad. Here in the USA OS is real good about replacing defective engines, you might want to send it back.