I'm going through the same on a 30 I have. Replaced diaphragms several times, changed the welch plugs, head gasket, even swapped carbs with a model J-A I have. Runs the same every time, runs at high idle and when I turn it down it sputters out. Tried adjusting jets throughout and nothing. My next best guess is coil/magneto but I don't have the tool to remove flywheel. Rings seem fine. I'd be curious about the carb you ordered. Chime in if it is of good quality and solves your problem.