Paul, you said the fitting with the red circle, and the yellow hose attached, just goes to the air cleaner housing. That is unusual - my first guess would be it is just a vapour line, for emissions control reasons. Normally odd bits of vapour are just allowed to escape to atmosphere. Since I don't know where it goes inside the carburetor, I can't guess whether it is doing anything unusual. What does seem odd is that it would have a serious fuel hose and a pressure-tight fitting at the carburetor end, for a vent line. The fitting that looks as if it would be for a vent line is the one in the green circle, which is clearly not a pressure fitting.
So, at this point I still don't understand the plumbing on that machine.
