Originally Posted by pelligrini
It sounds like a possible sticking float, or sinking. The needles might not be in the seats properly. They might have some pieces of dirt or something on them keeping them from closing.
I had a really old and troublesome 750 and one way I would dislodge a stuck float would be to open up the sight plug and lightly jiggle it with a small screwdriver. Be very careful doing this on a hot engine with the bowls overfull. Boiling gas can be dangerous.
Something else that hasn't been mentioned is your fuel pressure. You might just be blowing the needles off of their seats with excessive pressure.