If you cannot adjust the needles counter clockwise enough to remedy the lean condition transitioning to full throttle, you have partially blocked passages, debris in the internal carb screen filter, worn or disfunctional diaphragm membrane, an air leak, etc..... You may even have a crack in the carb spacer block. You won't find something more reliable than the Zenoah. This engine likely just has something overlooked, or easily missed. Is the engine overpropped with a restrictive muffler? How does the plug look? Are you using the older mag type ignition 26, or the newer EI 26?