Battery +ve to F1, F2 to A = Motor runs one way
Battery +ve to F2, F1 to A = Motor runs the other way.
You are reversing the flow between F1 & F2. These are the field windings (the coils attached to the body of the motor - i.e. the ones which do not rotate). A connects to the Armature which is the bit in the middle which spins via the brushes. The current goes in to A, into the Armature via a brush, through the coils on the armature, out of the armature through a brush and then to ground.
When the motor is running, A should be at about 6v. If it is much higher, then I would suspect the brushes and failing that the coils in the armature - but it could also be a short in the field windings.
If A is at 0v, the field windings have blown
Use the resistance setting on a multi-meter.
Test between F1 & F2 (with all the wires disconnected). It should read close to zero ohms (0.02 or so). Then test between A and the ground connection. This should also read close to zero.
Connecting the battery to any individual connection will not make the motor move. To A will make a big spark though.
Tell me the readings and I will predict what's wrong!
Si