You are describing the classic symptoms of using a tank that is too big, mounted too far away, and/or not turning enough rpm for the size or your carburetor. Use the smallest tank that you possibly can and mount it properly in foam. Then make sure the engine is turning over 8500 rpm, 9000 is noticably better for mixture consistency. Muffler pressure will also help. If you must use big props that lugg the engine below 8500, a venturi sleeve or smaller carb may be neccessary. There's always pumps and regulators but they can and often do open the door to tuning problems in other areas.