I remember reading on a thread somewhere about making sure the sides of the float dont have any shiny marks on them indicating that it is hanging up on the gasket that might be protruding from the mating surfaces of the carb upper / lower body.
Also - put an inline filter between the pump and the carb, this solved this exact problem for me when the little pieces of crud would keep the needle from seating - the pieces were small enough to where they were not obvious but out of frustration I put the filter in and the problem immediately went away.
