dirt man, in yo carbs , the fuel valve/seat is sticking open and when you tap it, the dirt is moved and it seals up again (then dirt finds it way back and causes it to leak)
clean the tank out with the pingle out and clean that too.
pull the carbs down, take the bowls off and the float, and clean everything with carby cleaner, then dry it off.
when you have the floats off, shake them near your ear to see if you can hear fuel inside (fuel inside means, throw them away).
then put your floats back on and adjust them to spec (i dont remember the stock float specs, ill find em when im home (20mm (.80") )
if you want to test if they now seal, get some fuel line onto the main fuel IN on the carb and blow into the line, when you push the float up into the carb, it should seal and you wont be able to blow air into the line and when you let the float move down away from the carb, it should let you blow all over its face....
if you can always blow into the line with the float up into the carb, you need to look into getting a new needle/seat.