G'day Mate,
Check for pin holes in ALL your fuel lines, sounds like it could be in the tank, as the fuel level goes down, the pin hole is exposed & sucks in air, thus leaning out the motor, also make sure that the clunk line has not come off inside the tank. Pull out the tank & check it, this happened to one of our guys last week, the motor would start but as soon as the nose of the plane was lifted, it stopped.
Good Flyin Mate,
Allan.