Negative incedince in the top wing causes it to stall after the lower wing. The lower wing is at a greater angle of attack and therefore stalls just ahead of the top one. The plane just seems to mush a bit foreward. My Goldberg is set at 0 for both wings and +2 on the tail. If you're going to set it for negative incidence be sure there isn't a warp due to the shim. You need to be sure the incidence in set in the center at the cabane and at mid panel at the interplane struts. This way it's nice and flat.