To me, it sounds as though the low end is rich. However, be sure the carb, and carb block bolts are tight. Air leaks cause problems like you describe as well. It may also be stumbling because the mixtures are different between the high and low end. If you look in the carb throat, you'll see that there are two holes in the throat of the carb. The first one is the primary idle, the next is the secondary idle. In the middle of the venturi is the main port (You can't really see this one). The two lower openings operate until you are in the top third of throttle travel. Mixture differences occurring between the secondary and main needles can cause burble and rough running. Get a good tach, and set the high end first. A setting 200-300 rpm rich of peak is what you're looking for. Then, start leaning out the low end. When the engine is warm, and you can advance the throttle quickly from idle to full without the engine stumbling, stop. Check the high end again and see how close you are to the peak setting you had earlier. You may need to richen it slightly again. Advance the throttle slowly from idle to full. If you can get from idle to full and not have the burble at the two thirds setting, you're good to go. A tach will make this easy.