Yes try the spray bar. You may also be rich on the low end. Inverted engines are more sensitive to a rich mixture. It could be your low was rich the whole time and you did not notice it until the head was tilted down. A hotter plug may also help with this. I have run several good 3D props on a 90. I like the 13x6MA for fast spool up on a light plane. I have also used a 15x4W. It's a good choice on a little heavier plane. It may also work well on the light plane but I don't have the ground clearance.

David