If you can access the carb intake, place some tape over it. Spin the engine over slow and watch the tape suck in, but if it blows out the reeds are warped causing disrupted airflow at idle speeds. Second guess would be a faulty, slow reacting spark advance in the ignition unit. Or the engine needs to be timed. Third, the throttle linkage might have a bind or hitch that a fast servo movement will overcome but a slow movment gets stuck on.