Here's an interesting article on the 1908 Union (SU) Carburettor.

"The Motor Car Journal"  May 2nd 1908 

https://books.google.com/books?id=KGIfAQAAMAAJ&pg=PA212&lpg=PA212&dq=union+carburettor&source=bl&ots=36OhmQgNzw&sig=75Ti_6Y_S25oSwLwUudl9WYQoaQ&hl=en&sa=X&ved=0ahUKEwjXipn438TRAhVBsVQKHbPkDT0Q6AEIQjAJ#v=onepage&q&f=true

Yes an interesting article indeed. Quite a simple but effective solution. Properly tuned with a stage 1 cam capable of 27 to 29 mpg. I can attest. They always work.