I have the following motor:
[GP307 Ultra Dash Motor by Tamiya](http://www.hlj.com/product/tam15307).

I've read online that you can give this motor up to 16 volts. I have been putting in 11 volts and it spins very slowly, maybe about 200-300 rpm.

I don't know much about how motors work, so I ask you, what would make one of these little motors spin very slowly?