RE: a 61fx thing
A) It's normal. Most engines have a "flat spot" near peak.
B) Weather changes from day-to-day, week-to-week, so the needles will change their settings. Air temperature and humidity will both influence the mixture. Since they change from day-to-day, you'll need to adjust them. Usually, though, the idle mixture doesn't need much adjustment, but to keep it "dead on", you will need to do a touch up from time-to-time.