Fuel level float inside carb fuel bowl is either set to low or the float is damage, gets filled with petrol and remains sunk. Therefore does'nt rise to cut of the fuel when the bowl is full.
If you lower the tank to below the level of the carb and it should stop leaking or at least slow it down, and will prove the point
Remedy, remove and strip carb, check if float is damage, if so replace with a new one. They are tupperware plastic and don't repair well for the labour and effort involved best to replace.
If not damaged then reset the cut off height, there is a needle valve facing upwards above where the float is but in the main body.
I have seen them refitted back without the needle valve being put back in at all.
Set to high and it cuts of to soon and theres not enough fuel in carb to keep it running at speed
Set to low and as you'v seen it over flows.