If you are still having problems with your engine, try a 17X4W prop.
I was trying to use a 16X6 and the plane did almost very well.
It would run great no matter where the throttle was.
But when I would glide in slow and then point it straight up and go full it would spit and sputter.
I put on a 17X4W and now it does just fine, now if I could just learn to hover.