[SFfiredog]

An engine idle of 850 to 900 rpm means that the throttle blades are open wide enough to let excess air pass the blades when you shut off the engine. This causes "run-on". Youy can install an anti-diesel solinoid on the carb which will close the throttle blades completely when the key is turned off. I would try to set the idle as low as possible to combat the problem. Also, heavier steel or cast iron flywheels can accsentuate the problem with their extra inertial mass. An aluminum flywheel can help cure your "running on".
HTH,
--Mike