I just had to add a photo I found from the summer of 1968.
I'm sitting on the 4-wheeler w/3 speeds my friends made.
You can see the idler pulley above the large pulley, the
engine pulley just above that. It even ran in the local
Hyde Park day parade!
My mini-bike was similar but not 3 speed and I had a
wheelbarrow 10" hard rubber wheel to the rear. Heck
of a rough ride, but it was real fun when we went out
on the roads 'en masse'. Fewer people to complain and the
police were more relaxed, but eventually, that fun was
restricted to our home streets....or else!
The photo is the best I can get. I hope you like it.
Wayne