Is the engine warmed up and at operating temperature when you are taxiing out? a bit lean on the low end can cause a transition problem on a "cold" engine, but it will go away when the engine is at operating temperatures, I guess it won't be considered lean at that point.

It is also possible that the diaphragm in the carburetor is getting some air pressure on it during flight causing it to go a little rich in the air, I had this condition on both of the rear carburetor engines I have, both were under a cowl.