[Barry_R]

Eljaro is correct.

Usually dieseling (or after-running) is caused by excessive closed throttle airflow. This is when the carb idle speed setting is too high. The fix for this can be more base timing, which allows for closing the butterflies up while retaining the same actual idle speed. One really light spring in the distributor can be used to allow a lower timing setting during cranking that instantly jumps up a bit once the engine fires up.