This definitely means your fuel level is going too high so here's some
stuff to check....
If both carbs are leaking, the line pressure is too high so, as Cliff
Hansen mentioned, you need a pressure regulator. If it's one carb or one
at a time....
1. Is the needle valve sealing? Remove the lid, blow through the fuel
inlet pipe and push the float up by hand-no air (breath) should come
past the valve.
2. Is the float pivoting freely on the pin? It should be quite a sloppy
3. Is the float floating?- extract the float pivot pin(with pliers, it
only comes out one way). Shake the float and listen for fuel inside;
look for cracks in the moulding.
4. Is the needle valve seating washer there?-unscrew the seat and look
for an alloy washer between it and the chamber lid.