Running On

Hi Folks - occasionaly our 1990 340 has a problem 'running on' after turning off the ignition - carries on going for a few seconds, then coughs and plutters to a stop.

Does anyone have any opinions on what causes this and what could possibly be altered to solve it (if there is any relevant variable it seems to depend on length of use prior to turning off and engine temp - although as a general rule I always let the engine warm up fully once turned on - but sometimes it can be warm, then stopped, partially cools and restarted, then if stopped again it might tend to run on as described)

Any advice gratfully received
Thanks
Dan

Our old favourite workshop manual lists cuases of running on as:
Faulty idle solenoid (this cuts off the fuel supply when you switch off the engine)
Idle speed set too high (should be 800-900 rpm)
Carbon build up causing ignition (dieseling)(can be caused by rich mixture)

General advice is check the ignition timing and the choke cable adjustment before even thinking about the carb. (carbs are often blamed for other components faults).

Anyone know better than this - get posting (please).