Terry_Robot:
Thanks again. Note that I did not understand why it did not work previously...
Servos are a negative feedback loop. To oversimplify slightly, they run a variant of this formula constantly:
drive = A * (setpoint - current position) - B * velocity
Where A and B are its internal control parameters (it may have some others) that define how fast it reacts, etc. When converting for continuous rotation, you are taking away the feedback (potentiometer) and lying to it about its position. By using two fixed resistors, the servo thinks that it is in a particular position and not moving. Therefore it becomes:
drive = A * (setpoint - some_constant)
And therefore you get a variable-speed motor controlled by the setpoint input.
Hope that helps...