RE: Air leak in break in
Sounds like the engine is revving ok. Check the slipper, check the grub screw for the rear output on the tranny, check the clutch shoes. It could just be an airleak seal the carb and back plate check the fuel tank seal make sure no air is getting in to the fuel line.
What gear ratio are you running? I know I said mine wheelied on break in, but dont take that as the norm.