When you make a turne you use the aileron and elevator that creates more drag on the plane = more load on engine, When engine starts to run out of feul it starts to lean out, If filter has a restricton it can cause a leane engine, Pending on the type of carb, that you may have restricted filter can cause leaning at mid range more so than at full throtel, The carb pump does not work as hard at mid range as it would at WOT.