Is this a sudden occurance or has it been getting progressively worse ?
What happen the last time you shut it down ?
There is a little solenoid on the bottom of the carburettor, it should go on & off as you turn the ignition on & off.
Some tmes you can hear it click but I like to hold it or even remove it just to be sure.
This controls the fuel to the carburettor so when you remove it fuel will leak out and if you crank while it is not in there fuel will gush out.