I bought here a LIFAN scooter, used. Two days ago I refueled it. Now he spits constantly fuel from the vent hose of the low pressure gasoline tap as soon as it runs. I can exclude the low pressure gasoline tap, I changed it. What can this be? I also have the feeling that the engine occasionally stutters while running. The tank has only one outlet for fuel, no tank ventilation as a connection (then goes over the lid probably me?)