As it has a fixed pitch prop, the amount of power produced static and airborne are almost the same, the main difference being dynamic air pressure on the air filter, a marginally higher RPM and improved engine cooling.
If I recall correctly, when I worked on light aircraft, we had a Piper Warrior with similar problems, yes I know different engine and all, but a similar problem. We tried all sorts to fix this beast, went down the route of sticking valves, carburettor float and main jet etc, it turned out to be an air filter problem, although the exact filter problem escapes me, it was eventually resolved by replacing the filter scat hose and element.
In summary, what I am trying to say is that the problem is related to either air pressure or flow.
It's been a while since I worked on light aircraft so I wouldn't like to guess any further without direct involvement. If i do remember anything similar of the problem you are having, then I'll let you know.
This is definitely, As I See It