trisager wrote on Aug 18^th, 2010 at 9:16am:

I don't believe the 707 had an apu, at least not the early ones. According to the manual (still on the fence about buying this one, it is a bit expensive for me), you can connect external air via the animation control panel, try that for engine start.

Thanks for that.
Unfortunately I have tried everything (almost^*) I can think of so far, which includes via the External Air being ON. I'm just not being very successful. I only get it started about once in every four tries and and so far every time I have it's been by using CTRL+E.
^*Except the one that works!

I did go to the 707 - General Discussion forum and found QID 203, but only managed to start engine #3. Engines #4, #1 and #2 refused to start. In the QID 203 it says to turn on the APU IF available. I really believe that there is some switch that is either ON when it needs to be OFF or vice-verse (it could be more than one). I'll keep on trying, but if worse comes to worse I can always start the flight with the engines already running.

Mark