You should measure the full header pipe length to where it meets the beginning of the can where it starts to increase diameter. I made a boo boo and provided the wrong method earlier. A lot of planes have tunnels for cans. That didn't mean they had the tunnels and engine mounting locations that would permit effective use of them.

Just an FYI, the prop used will have quite an impact on pipe/can performance.

While you'e working with the above, check for leaks around the reed block and carb base. It possible there's an issue there that's contributing to the problem. The 106 is a very good engine when all is as it should be. I seriously doubt there's anything mechanical causing the problem. It can only be carb and pipe related.