This is by far one of my favorite DIY projects. I have yet to do this, but I think that it would be good to know how to do in a pinch, or to just keep in the garage in the event you needed to pull it out and utilize it. I have seen multiple versions of this plan online but came across this one today and figured I would share it with you: http://goo.gl/XEx4
Here is a PDF I have found as well: Build a generator from a lawn mower motor
Pics of a Lawnmower Generator found here: http://goo.gl/Nc7t
