Hi Garry,
I did route 66 3 years ago and its a cracking trip (as a lot of the touring holidays are). My trip was the first year JY did route 66 and I believe they have tweaked it slightly since then (for the better).
Any specific question please ask.
September is a good time to visit the States (if you avoid the gulf coast), you should still get some warm weather although some places may need a light jacket or fleece in the evening (possibly Chicago due to its location and Flagstaff due to altitude).