You have probably checked these, but it may be worth confirming or revisiting:
Are you sure the exhaust pipe is clear? I routed out mine to increase flow.
The original air box is limiting. I replaced mine with a cone shaped filter and it helped.
Are you premixing gas 50:1? I would always premix and not use the oil pump. The pump is not always dependable, and adds more oil than needed (forgot the ratio it produces) causing it to be less rich.
Are you sure you are mixing gas-oil correct? I was refilling the little 2-stroke containers from a quart and later realized I was using too much oil. That will clog things and slow it down.