For now-BOTH!. Assuming that you are transporting the kart horizontally, this should not be happening unless pressure is building up in the tank. Pull the carb off. Check for the floats being installed properly and there isn’t any fluid in them. The “alto” stamped on the floats should be facing up when the carb is in its operating position. You should be able to hear them rattle when you shake the carb up & down. Also check the needle valve for debris that could be preventing from shutting the fuel supply off or maybe the needle and seat just needs to be replaced do to wear and age.