I would suspect that maybe one of the following is occuring to have the fuel leak like that.
Worn or dirty float valve seat. (In that circle shaped bowl on the bottom of the carby, there is a float with a little needle running off that, its possible there is a bit of grit on that needle or its seat and allowing fuel just to keep running in.)
The float has a hole in it, (Once again in the bown under the carby, the little brass coloured float may have got a hole in it and sunk to the bottom aloowinf more fuel to rush in.
Lastly, maybe the choke is stuck on, floodying the carby and it is running back?
