Those are to be connected to the two nipples between carb 2 and carb 3. The diagram doesn't show them very well and that they're named
TUBE, FUEL isn't very helpfull either. They are
not for fuel but are there to vent the carbs. The Parts List of the CB500K2 (ED, F, G) shows them better: number 14 in the pic in p.32 in:
http://www.honda4fun.com/pdf_parts_list/pdf_spac500/CB500-76/CB500-76_2.pdf They're supposed to have a length of 330 mm and a ⌀ of 3,5 mm. They're to go under the airfiltercase where the other 4 (overflow tubes) go.
Sorry I couldn't answer you rightaway, but I had to attend a march.