It flew OK 2 months ago and it still makes maximum RPM on the ground.

If it was the prop, it wouldn't have flown OK previously. If it is too rich, it wouldn't make maximum RPM. Also if it is too rich, once in the air it would lean out and run better.

The only thing it can be is too lean. Run the engine at full throttle, then point the nose straight up. If the mixture is correct, you will hear the engine RPM increase slightly. If it is too lean it will die off.