In 35 years of R/C flying I have never set up a tank so that the clunk moved to the front of the tank. If someone was to do so it will get stuck there. The fuel will be moving around in the tank enough to keep the engine supplied provided it is not set too lean. Holding the nose down while the engine is running is a useless test. Set the tank up with 1/8" Tygon tubing leaving about 1/4" clearance between the clunk and back of the tank and then set the needles to manufacturers recommendations. This is not a glow engine so not a big RPM difference between rich and lean so I am sticking with my original assessment that it is just set too lean.