[DanEC]

That often happens when the idle speed is to high or the carb is not set up right. At idle the front throttle plates should leave 1/2 to 1/3 of the transition slot exposed in the bores. Has your carb been worked on recently or have you increased the idle speed lately. As a work around until you fix it - before turning off the key start letting out the clutch and when the idle starts to decrease, turn the key off and let the clutch out the rest of the way all at once.