I'll answer your riddle with a riddle. Why did the spark advance change in your experiment "when the VSS signal shouldn't matter"?

dave w

I once read somewhere, don't remember where, that the IAC resets at 35 MPH. It's only here say information that the VSS is needed to reset the IAC.

Maybe someone else can confirm or deny the VSS...

Sometimes the IAC adjustment procedure in the attached .pdf will help idle.

dave w