Hi,

Looking at some of the parameters of the motor:

Free-run current @ 6V: 50 mA

Stall current @ 6V: 710 mA

and

Free-run current @ 3V: 40 mA

Stall current @ 3V: 400 mA

This means that average current draw under load should be around 380mA on 6V and 220mA on 3V (ideal).

Calculating for this...

The motors start up resistance is probably around 8 Ohm (+/- 0.5 Ohm).

A 330 Ohm resistor in series means you make a voltage divider which (at the instant you apply the voltage) will be: 9 * 8 / 338 = 213 **m**V (0.213V), which won't make it go of course.

A more suitable resistor would be close to 8 Ohm (2W) for 6V to the motor, as the, motor will look more like 16 Ohm when drawing 380mA (peak efficiency).

You won't get it running like you expect on a 9V PP3 battery, as its internal resistance is too high - 4 AA Alkaline cells in series will give you 6V and then you don't need to add any resistor.