Start it upright with the wings on, then turn it inverted. That is more the actual flight configuration than running it inverted without the wings. Are there any fuel lines, including vent lines, that are in the area of the wing that could be pinched when inverted by the wing being on? Have you checked the whole fuel system, especially the clunk in the tank, for differences in it between upright and inverted? Is the tank securely fitted so it does not move around? Maybe try the regulator cover vent hole tube to inside the fuse? That is the one thing affected by airflow differences. Good luck.