That line with the fill plug in place appears to be the filler line to the tank. I think that you need a vent to the tank to keep vacuum from overcoming the pressure-pulse pump's ability to draw fuel. On non pumped engines, that is accomplished by the line from the muffler. Did you completely tear your carb down? If not, you may have a piece of trash hiding under one of the needles. I had a supertiger 90 several years ago that would starve itself for no explainable reason. Finally after about the 10th tear-down I found a very small grass seed hiding in the High speed needle spray bar.