Disclaimer: I rode Route 66 in 2015, so this information is nine years old. I don't really know, but I somehow doubt that much has changed in those nine years (except the world has gotten hotter).
I rode from Needles to Amboy on September 19, and Amboy to Newberry Springs on September 20. I left Needles at 5:30 am and arrived in Amboy in the middle of the afternoon. Yes, of course it was hot, but it was only 103 degrees when I arrived in Amboy, so it's unlikely to kill you unless you didn't bring enough water or don't drink enough of it (take multiple gallons). Note that there was nothing in Amboy in 2015, not even water (there was a gas station that sold gas for twice the going rate, but only during the day). The town of Amboy died a long time ago. So you have to take enough food and water for two days. It's quite heavy but doable. And it's unlikely that a passing car will be able to rescue you, because there are no passing cars. But you can pitch a tent pretty much anywhere.
After getting to Ludlow, I rode the shoulder of Interstate 40. Because of all the exploded truck tires littering the shoulder with tiny wires, I got more flats on that section than I have gotten in my other 20,000 miles of touring. Be sure to take tweezers, as truck tire wires are hard to remove from your tire.
Most of the bridges in the Mojave Desert are over dry washes, so it's usually possible to get past a bridge that is out by going through the wash. And there will be washed out bridges. And the road will probably be closed, but you can probably ride it anyway. Get local information before proceeding.
By the way, I really enjoyed my ride through the Mojave Desert.
