Seen these questions asked a few times on social media, such as, how does the belt run on a old Hayter Harrier 305, 306,307 etc.  So I've made a couple of diagrams that might be of use.  They are not to exact scale but you should get the idea. 

The belt Run.



The Belt Idler arm.

A lot of people seem to be unsure about where and how the arm return spring fits, so this diagram should help with that.