Usually with a tuned pipe, you have the problem of the engine unloading in the air and rev'ing up more than it does static on the ground.
So it may be leaning out in the air on you.
My old method was to tune the engine on the ground with a prop with 1 pitch less than the in the air prop I use. For example if I used a 11x7 prop in the air, I would tune it with a 11x6 prop on the ground. That would get me closer to the needle setting needed for in air flying.
Later some engines had a second servo operated in flight mixture control that one could use to tweak the engine in the air too.

But in any case you may be a just little too lean when the engine unloads more in the air on you than it does on the ground.