It could be overheating in the air, and quitting on you.

Set the main needle 4 clicks richer than you normally would
and try it. Give the motor 2 minutes at full throttle in the
air (allow it to get hot) before you pass judgement on it.

If it's not leaving a slight smoke trail, bring it down, and give
about three more clicks. Sometimes a motor has to be set
really rich on the ground, to prevent it from overheating in
flight.

Remember that the mixture is the only thing that will keep
the engine from overheating, and quitting on you. Hopefully
you are running a castor blend fuel in that motor.

Dave.