It looks like you have an engine problem, but you haven't given much info about it. Is it new, broken in, yet? I suggest get the engine sorted out before you do any airplane work. Is your idle set right? Often, it is difficult to get the low end lean enough with the air bleed carb, but be sure the engine has been run in before fussing too much with the low end.

I agree that under best of conditions, you may find that plane underpowered with that engine. It is a great engine, but it may be a little overtaxed with this airframe.