From the 750 'archives' where this happens: it is possible for a sharp edge on the float tang to scrape up a tiny ridge (on the float pivot supports) that interferes with the float movement. It then usually holds the float valve slightly open, letting it weep fuel in until the carb overflows. This happens most often on the sidestand: on the centerstand it only happens once in a great while, or not at all, and when riding, not at all. This is because the tiny ridge can't hold the float tang very well when the bike is moving about, so the carbs behave while riding or on a centerstand, but weep on the sidestand.
So...check on the insides of the float pivot supports where the brass of the floats rub them. See if a tiny ridge might be raised there, and gently file it smooth again. Maybe also debur the edge of the float bracket while you've got the file out?
On the 750 this is legion. Its symptoms are almost always: it leaks fuel while on the sidestand on hot days, even if the petcock is off (it only leaks until the fuel stops expanding, like when the engine cools off or the fuel runs out of the fuel lines). It doesn't leak fuel if on the centerstand, or running down the road.
