Universal motor as DC generator
A series wound field generator needs not only a load to maintain the field current while it's running, but also enough residual magnetism in the iron to 'self-start'. 

The amount of magnetism needed is dependent on the conductivity of the external circuit. Unfortunately the better the motor, the more likely the iron is 'good' for motors, which means very soft, with little residual magnetism.

One of your options is to whack the field windings with some DC to magnetise them as far as they can be. Then try starting. They may have stored enough, they may not.

Another option is to disconnect the windings, and separably excite the field with a power supply. Of course, once the generator is running, this can be powered from the output, so not much external power is needed for starting. This is not over-unity, as power is being supplied to turn the generator.

Another option is to connect a battery briefly to the field when starting, perhaps through a diode, to kick-start generation.