Check your plug. I bet it is black. Lean out the H needle. your rpm should be well over 9000, which is over the top for this engine, but OK for 3D.
BTW, 3D will be better with a larger, but light weight prop like 18x6 or 18x7
A normal fight prop will be like 17x10, 18x8, 20x6, all depending on the plane it is in.