Either your seat is worn and the flow needle gets jammed up and can't seat properly. Or you have contaminants (Debris) in your tank and its getting sucked up and stuck in the float needle. Are you cleaning the carb and finding crud in it? Does the seat look out of round? Slightly squared off? I would do as fishkiller said and replace the needle n seat. I would also check your fuel tank for sediment at the bottom. And also install an inline fuel filter.