Density altitude and all the things that go into that equation can have a huge affect on airplane performance. Anyone that has ever tried to get off the ground in a heavily loaded J-3 on a hot summer day would give witness to that for sure!

But removing most or all of the variables, humidity by itself can play a huge roll in engine performance regardless of what that engine powers. If you ever race anything, you soon learn that with all else being equal, water in the air can cause huge changes to engine tune. Add water to the air and the engine goes rich as there is less oxygen to burn. Remove water and engines go lean as you don't have enough fuel for the available oxygen level. High performance race car engines can go so lean as to cause damage such as burned pistons. Our RC gas engines are no different and major changes in humidity levels will require some needle adjustments. Performance changes are normal in this case.