Snow blower motor backpedaling?

[lphung]

Hi,

I am a mentor of a rookie team this year and we tried to use the snow blower motor from AndyMark to drive an arm on our robot:
http://www.andymark.com/Motor-p/am-2235.htm

However, the load on the motor seems to be too heavy and causes to motor's shaft to turn in the opposite direction of our desired input. In general, I do not expect the direct transmission to be reversible with worm gears (I would even suggest that if there is no inputs on the worm gear, the output shaft should not technically move). Do you have an explanation regarding why the motor would be "backpedaling"?

Thanks.