If there is not an inline fuel filter I would stick one in there because it sounds like something be it rust dirt or paint found its way to the carb. before getting to involved have you tried any fuel system cleaners (personally I trust sea foam because it has served me well in the past) Also because I like to take the lazy approach first and then assess whether more work is needed I have had good luck with this type of scenario by draining most of the fuel from the tank and letting the bike run and burn through a good pint to a quart of 99% isopropanal alcohol (if you cant find that cvs or walgreens will usually carry a 91% version but then with the next tank of fuel it helps to add some fuel system water remover. The whole process won't cost any more than $10 to $15. your not out much and it very well may work for you.