Rough running at idle and low speed operation most often occurs when the pilot circuit in the carbs becomes partially clogged. The engine will also experience a miss or throttle lag from not getting the correct amount of fuel.

You can do a little test to determine if that is the problem. With the pilot screw tamper caps removed, turn the pilot screws in a turn and check to determine there is any change in idle RPM. If not, that could mean the pilot circuit on that carb is partially blocked, and possibly even fully blocked.

The corrective action would be to remove the carbs, remove the pilot screws, and fully clean the pilot circuit including the slow jet. If you know the carbs have been sitting for a long time we advise that you rebuild them, which will often save some time and aggravation in the long run.