Everytime I shut my engine off, with mixture to lean, my engine shutters pretty strongly.  It is acting like the fuel doesn't completely cutoff so a cylinder or two might be firing still, like the mixture isn't totally cutting off the fuel.  It happens the same identical way everytime..  It doesn't appear to be post ignition.  The engine is a Lycoming O-360-A1A with a constant speed Hartzell prop. The engine has 290 hours and is in a 1975 172M.  Ideas?