mate of mine had the same issue but turned out the carbs were set real rich and it was using more fuel then the pumps could supply, as you have tried 2 carbs im guessing 2 of them can't be set wrong. my ute also had a similar issue as the pump is on the engine, one of the hose clamps had come undone and it was drawing air in the hose with a little fuel so in saying that maybe a lose hose clamp or a small hole in one of the hoses between the tank and pump.