My understanding on Petcocks on Hondas is that there is 1 fuel line (1/4" to 5/16" ID)
and 1 vacuum line (3/32" ID). The 3rd. port on the petcock is left open to atmosphere.
In one case, I saw a filtered vent line on that 3rd. port, but it was still open to the atmospehere.
The fuel line connects to the floatbowl (s) and the vacuum line connects to the engine intake at 1 of the carbs.
From the drawing, I would guess that even though it has a hose on it, the outermost small nipple should be
vented to the atmosphere, not connected to the engine intake.
Note that the left most hose has only 1 hose clip on it and none on the "open" end.