At that age, wear on the fuel needle valve in the carb has probably taken place. Leading to a tendency not to seal every time and shut off the fuel to the float chamber. Two options. If you are happy to strip the carb and have the tools, remove the carb, this may be more complicated than the carb itself!! Remove the screws holding the lower casting on the float bowl and carefully remove the float and needle valve. Look at the tip with a magnifying glass and see if there is a wear ring around it. If there is then you need a repair kit for that particular make / model / chassis number / year bike from a main agent or an online parts house. In addition you may need 2 kits because the other carb(s) will be in a similar condition. If you aren't sure what to do get a good bike mechanic to guide you the first time or a good manual. If the mileage is low, there may only be dirt in the carb(its 18 yrs old) from the tank and a flush through with carb cleaner followed by a blow out with an airline. I doubt its the tap.