If this is a gasser with a Walbro carb, you have a carb problem, not a tank problem. Gassers tolerate high or low tanks very well and gas can not siphon through the carb if all is right in the carb. The most common cause of this problem is an improperly installed regulator diaphram that is holding the needle off the seat allowing gas to leak through the carb. Depending on the carb model, you could either have the diaphram installed with the gasket on the wrong side of the diaphram or in some carbs, the needle fork has to attach to a slot on a nub in the center of the diaphram. If positioned on top of the nub rather than in the slot, a leaky needle is almost guaranteed. Another cause could be a bad needle or seat or trash in the needle. Usually when this problem is present, you have to have the H and L needles screwed in more than normal just to get the engine to run.