wall wart powered motor

I have attempted to attach a 12 volt motor to a 12 volt power supply (wall wart) from a router. It keeps pulsing on and off and wont run continously. The arduino is not involved at all, just a direct connection from wall wart to motor. Why does it keep pulsing and is there some kind of filter I can attach to it so it doesn't pulse?

Sounds very much like the motor is taking more current than the supply can produce without tripping out. Motors take a LOT more current from standstill than they do once running. A motor with a 0.5A no-load current might briefly take 10A at start up (or try to), for instance. If the power-supply is switched-mode it might well go into a fault-state if overloaded and temporarily shutdown and restart - hence the pulsing.

Details of the motor and supply will be useful, we can't read minds.