Some modern engines just won't run for beans without the pressure line. The needle valve holes are sized to require the pressure. When running with pressure its common to need just 1 1/2 turn open needle... without pressure the needle needs to be about 3 1/2 turns open. (if the "jet" hole is big enough for it to work)

You don't have to buy the expensive fuel valve... You cn use the old 3-line tank setup, with the 3rd line being your fill, and just have a plug for that line ( a 1/4 inch piece of hevy wire works for the plug) Some people will put a second clunk in when doing this.. that makes it easier to remove fuel from the tank.