All I know is that this is definitly not normal! Set both the high speed needle and low speed needle to the factory spec and if it still does the same I would think it is an air leak either in the fuel system or a carb seal. I had a similar problem with a new .15 Associated engine and I just sent it back to the factory and they sent me a new one!
Other than that I cant think of any other problem that would make it lean out.