How to increase the torque of a toy motor?

I got bored and wanted to mess with one of those nerf guns. The six shooter revolver kind. I bought a motor (the size of a flash light battery with a little stick hanging out that is the axel so you know what I'm talking about.) The idea is to tie a string to the axel and have it spin. connect the other end to the plunger in the dart gun,(it's set up like SPRING/ PLUNGER WITH STRING/ OTHER PLUNGER/ EXIT.) and pull it back to fire automatically. however the Motor doesn't have enough power to pull it back. my uncle told me that the motor was to fast and I need to use the speed and turn it into torque. He said that this could be accomplished with pulleys. So would rigging up a pulley system to pull back the spring work or is there some other way? It's a revolver nerf gun so I'm going for semi auto fire while keeping it small enough to walk around with. I saw another semi auto mod but that used a tank of compressed air witch Is why thought of using motors instead.