The line from the front of the petcock, the one in the pic that goes to the fuel filter, which I would remove for now, is the fuel out to the carbs. The other line goes to the vacuum source, which is what appears to be where the filter is connected. I just went out to check this on my running Ninja 250. The original line that supplies the carbs goes to the front, larger fitting on the petcock, and it goes straight back to the inlet of the carbs. I'll see if I can grab a pic in a minute, after dealing with a cranky 4 year old...