hi their i have a 50cc mini moto/fat bike and it wont start ?

hi their i have a 50cc mini moto/fat bike and it wont start i have tried putting petrol down the head and it fires up for a second or two then dies what could it be

i have putted a new spark on it and ignition coil and the carb is nice as clean what could it be?

DAVE
February 2013