My first though was that you might have ignition problems, like worn points leading to incorrect timing. Then I remembered that 1220's have HH120 engines, which are solid state ignition, so no points to get out of whack.
Yea Im pretty sure its the carb. (my carb guy thinks so also) I am no good with carb adjustments AT ALL I just dont seem to have the touch. I should know later today he took it to work with him to clean it he works in a mower shop and they have some sort of sonic cleaning tank they put them in. says it works great and usually dont have to pull the carb apart.