I've watched people change drain valves (like the Curtis) by simply letting the fuel pour out while you quickly insert the new one. This seemed a bit silly to me, not to mention a tiny bid dangerous. In the past I have drained the tank to do this, usually in conjunction with other things. However today I was in the process of checking my fuel vent line and the obvious idea of simply plugging the vent line, putting the fuel cap on and letting suction in the tank stop the fuel from flowing while you change the valve. Anyway it worked like a charm. Just put a bit of hose with a hemostat or tie-wrap to seal it over the vent line, put the fuel cap on tight (has to be non vented type), then drain a bit of gas out of the old valve and presto, it stopped after about a 1/8th of a litre. Undo the old valve, put in the new, unplug the vent line and your done. No fuss, no muss, and much reduced danger.