What is the concept behind speed change in a constantly loaded DC series motor?
Neglecting voltage drops across field and armature resistances, and assuming supply voltage is not changed:- 

The diverter circuit cuts field coil current in half, thereby halving magnetic field strength, doubling Kv, and halving Kt. 

Doubling Kv causes speed to increase to 1000rpm. As load torque is proportional to speed squared, torque will increase by 4 times. Since Kt is half, 8 times more current (160A) is required to deliver this torque.