I would stick with the 9x6 with a .40. The 9x7 sport prop will be a little too heavy (we don't even use them on .46 FX's racing). You may want to play with some of the lighter pylon props:

8.75 x 8.5W
8.75 x 8.75 W
9 1/2 x 7 N (This is actually lighter than a 9x7 sport prop)

My guess is your either getting fuel foaming or an air leak in the fuel system. This engine should have no problem running in the 16-17K range.