Hi Niclas
My Graupner Harpoon was completed last week and I have already logged 21 flights on it.
Though I did not not notice the same problem as your Harpoon, when the vector thrust pipe was installed per instructions, the engine did not end up in the center of the pipe. We had to glue a
3/8" hard wood on the mount on both sides to drop the engine down so it would center itself on the pipe. PLus the servo for the vector thrust pipe was installed on the provided metal bracket on the left side of the engine with the linkage going from the side and under the engine.

Happy Flight
Jason