If the engine kicks over and starts for a brief interval, then dies AND you have the needles set as per the factory specs... then choose any of the following

1 - The carb is not getting fuel or enough fuel.
2 - The tank is not getting muffler pressure
3 - There is an obstruction somewhere ( see 1 )
4 - There is a leak somewhere ( see 1 )
5 - Your lines may be too long
6 - You may have your lines reversed
7 - The clunk line may have fallen off within the fuel tank.

Open the needles about 1/4 - 1/2 turn more than factory specs to start with.