Could be several things:
1. Trash in needle & seat area -- remove needle & seat, clean, blow w/ air & replace; may also be worn and need a new needle & seat;
2. Improper float height -- reset per manual ( I can't remember off top of my head here at the office)
3. "Wet" float -- As floats get older or develop hairline cracks, they absorb or hold fuel inside, making the float heavy, therefore it won't rise properly and seat the needle, thus fuel continues to come into the bowl.
4. Testing: remove carbs & drain fuel. turn carbs upside down and put hose onto fuel inlet. Gently blow thru hose, it all needles are seating properly, you should not be able to pass air thru the hose, unless you blow on it substantially. If air is passing, gently hold the floats for #1, # 3 & #4 down with your fingers and test only the "trouble" float. If it still passes air when you push down on #2 float, then the needle is not seating. If air does not pass, the needle is seating, and the problem is likely the float is "wet" or improperly set.
I'm sure there are other and better ways, but this is how I do it. Hope it helps.
