Yes, it is normal for it to lean out as the tank gets lower - ie the temps will go up. Sometimes you have to change the tune from day to day (that also depends on the engines...some "hold" a tune better then others....ie on some carbs, the hsn and lsn will vibrate and move a little....that is why OS engines "hold a tune" so well...the needles do not move). Humidity changes will also have an affect on your tune....stay with it...you will start to feel what needs to be done