Getin started problems

Hello to those who are reading this I Have just built my first motorized bicycle and it was pretty easy to put together but I Cant get it started I point it down a hill hold down the clutch and pedal then let go of clutch and it the motor spins a little and then it dies gets me about ten feet : I have GT2B 48 cc Sky Hawk If you have any Ideas I willing to try anything so far

Thank YOu,
programmer1200