[DanEC]

Besides timing if you have a large cam and to compensate the carb primaries have to be opened way up to get it to idle, it will often diesel when shutting it off. Assuming you are carbureted, are your idle mixture screws responsive? If not the primary throttle plate may be opened above the idle fuel slots which is never good. Setting the idle speed up high can do the same thing. Sometimes the secondary plates can be adjusted to pass a bit more air at idle to compensate, allowing the front throttle plate to be closed to where the idle mixture screws are functional. The screw to adjust the secondaries is underneath (have to remove the carb). Just saying as sometimes a little careful tuning of timing. idle speed and the carb throttle plate can take care of it.