Your low speed needle valve may be too lean. If the tank is slightly lower than the carburetor, the fuel will flow back into the tank when the engine is off. This is normal. If the engine is tuned up right, it will start on the prime and then the carburetor will keep the fuel flowing.

But if you are starting at low throttle and the low speed needle is too lean, it will burn off the prime and quit.

Try starting it at one-third throttle or even full throttle. If the HS needle is open 2.5 turns it should start even if the low speed NV is off.