Problems w/ 9-18VDC motor

I'm trying to run a 9-18DVC motor I bought at radioshack. To power it, I used an AC adapter I had lying around. i cut off the non-wall end of it and soldered the lead and ground to the motor's terminals. the motor runs fine but starts to smoke after about 30 seconds. Any idea why this happens or how it can be affected?

The adapter label says its outputting 16VAC at 1000mA, but why/how could it still be putting out alternating current? I thought the whole point of these adapters was to change to direct current.

No. Their primary purpose is to change the voltage level. AC to DC rectification is not always included on low quality wall-warts.

As for the motor, it's likely just overheating from being driven forward and backward constantly.
Either place a diode on one of the leads or build a bridge rectifier.

Lots of Wall Warts output AC