Swap the carburettor or take it apart, clean it all, check the O-rings and put it together.

One time I had such an issue and after some trying I took the carburettor apart and a small piece of the fuel line was in the housing of the main needle. Probably it snapped off when putting the fuel line over the tank nipple. I must admit I did drill the nipple up to a slight lager bore giving a sharp edge on the nipple