First thing I would try is increasing the idle speed with the two screw stops for the linkage.
On ZS carbs pulling the 'choke' does two things - slightly increase the idle (uses a cam) and slight lift the carb pistons (uses the start bar - a shaft with a flat on one side).
To set the idle I would first assure the carbs are balanced (adjust them w/ one of the screw/bolts on the accordion connector loosened) then tighten it up when they are balanced and the idle speed is good.
Beyond that there are many things you might check - timing (changing timing due to points wear or slipping), examine carb diaphragms, mixture, float level & float condition, etc -- IOW a complete tune-up.
But if the problem is simple and slight an idle adjust may get it done.