What is it mounted on, test stand, airplane or what? How are you priming it?
You are sucking air somewhere. Assuming the tank is airtight and hooked up correctly
the first thing I would do is change the fuel lines, the next thing is to check that the carb is seated firmly onto the engine and locked down tight, next check the o-ring on the needle valve and make sure it is providing a good seal.


Regards,
doubledee