In addition to what tIANci said, there's a common test to confirm it's not too lean. Once you get the high speed set slightly rich, hold the plane firmly with two hands and point the nose straight up, and keep it there for 5-10 seconds. If the engine speeds up when you first point it up, and does not begin to slow down, it's not too lean. This test also helps you confirm the engine will draw fuel through your manuvers.