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One way to 'test' a series DC motor is to connect A1 to D2 with a jumper wire (say #4 or so) This connects the missing part of the series. Connect a 12v battery to A2 and D1, and the motor will turn. (you may want to strap it down do it doesn't roll off somewhere due to torque...)
Here is a diagram - The motor has two basic parts - the stator (D1 and D2 are stator end connections) and the armature (A1 and A2 are armature end connections). In a series motor, they are (as you would guess...) connected in series. So ->
+ A2 ~~~~A1 <jumper wire> D2 ~~~~D1 -
The reason you created sparks when A1 and A2 were connected was that you were running the battery power through the brushes, and through the armature (essentially a short). Same reason with D1 and D2 spark - 'shorted' both ends of the stator. Both need to be powered (in series) to make the motor turn.
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