Fuel dribbling out the lower fitting on the carb while starting? That sounds like too much pressure in the fuel system. I see there is a tiny vent in the fuel tank cap, likely not enough for the size of pumps used on shifter karts. Guessing your pump is a 52mm Mikuni, can’t see it in the pictures. Did you check the needle & seat carefully for any sign of wear? If it’s leaking, that could be part of your over-rich condition.