sounds like the float valve is sticking or needs replacement , turn the petrol tank off and drain the carbs, using the drain screw that you said was allready tight, stick a little pipe on the carb bowl and empty them.

its a case of whiping the carbs off, removing the float bowl and checking the valve, if it looks ok ie tip not a sharp even profile they may need replacing or more commonly they need a clean and the float height checking, best descibed by the manual. basically the float bowl is overfilling and petrol is escaping via the bowl vent.