I'll second the prop change idea. That 3-bladed prop is really not very efficient. Just about any reasonable size conventional 2-blade prop will give you more thrust and get you off the ground faster. 12x4 would be good, or an 11x5 for more ground clearance, or even a 10x7 works well. Two of my students have your setup with 10x7APC props, and man, those planes get off the ground in a hurry.