It is also possible that something is interfering with the seal between the needle valve and seat. This could be foreign matter such as dirt or rust which has accumulated in the tank.
The fix for this particular problem would be to clean as much residue from the tank as possible, then disassemble the carb, cleaning both the needle valve and seat. Reassemble the carb and install a small universal fuel filter between the tank and carb. If the needle and seat assembly have not worn excessively, this should solve your problem.