sounds like a sticking needle valve. if the bikes been left for some time, with fuel in the carbs it can gum up.you will be best off stripping the carbs and give them a good clean with carb cleaner.also check that the floats do not have fuel inside them. its never good to let a bike stand for any length of time with fuel in the cards. i shut the fuel tap and run mine untill the carbs are empty. you should turn off the fuel tap when you park up anyway, as its not a vacuum tap.