This goes way back, some 45 years or so, about the time I first rode a motorcycle. It was a Yamaha 125 enduro model if I remember correctly and would have looked something along the lines of the bike below. The bike belonged to my father, who taught us kids how to ride.
The photo above was taken from the wanderingalaskan.com travel blog. I chose to use it over the hundreds of others found on the web because of the story behind the bike. Maya purchased it, not running, as a project bike. She took it apart and put it back together to learn how it worked. Here is the story as told by Maya.