Switch to a 13x6 and all will be well, the airflow velocities through the carb are too low with that much prop, this makes the fuel draw too low so you compensate by setting the high end overrich at full throttle. Then when you go to part throttle where the fuel draw improves it is too rich. If you really need to load it that heavy, it needs a Cline regulator. You're sort of asking it to be a 100.