Shut down the stove immediately as you have a very serious and potentially deadly problem
. CO is nothing to mess around with as it is a colorless and odorless deadly gas. No coal appliance should ever be operated in a home without at least 2 CO detectors. One should be placed in the room with the stove and opne outside the sleeping area. What you are smelling is sulfur dioxide which is another by product of the combustion of coal. THis flue should never be present in the home and signals you have low draft situation which can be caused by a number of things ranging from to much combustion air, an incorrectly set baro damper to a poor drafting chimney. If you don't have access to a manometer (draft gauge), then you need to call in a professional. Was the chimney cealned after the wood burner was removed, if not that would be a start. However the draft needs to be set with a gauge for proper operation.