It sounds like your fuel tank might be a little too high in the fuselage. Try setting the needles as you described, but at 1/2 tank,. when its running good, then shut down, fill the tank & see how a higher fuel level effects it. if tanks too high, it will be too rich on the high end & a little bit rich on the low end. The center of the carburators spray bar should be about 1/4-1/3 of the total height below the top of the tank. When its lower than that, the engine is not really pulling the fuel as much as it is when the level drops. At full tank, the fuel free flows to the engine & the needle setting is restricting that free flow, but as level goes down, then the engine has to pull the fuel up to the engine & through that needle setting, it is too lean to do so. Bill