Luis,
The problem may be in the float valves (conical black things that go into that well).
That black pointy end is made of Neoprene, which gets harder with time and exposure to air (when carbs are dry).
The solution to that is replacement of those two.
The bottom of the well against those valves close sometimes develops sediments from fuel and dirt that avoid a perfect seal.
When the engine is working, the vibrations make the harden valves leak.
Follow these links for cleaning and adjustment:
http://faq.ninja250.org/wiki/How_do_...n_the_carbs%3F