When central is turned on there is a foul odor

Is the smell coming from the registers?

You have dirty sock syndrome. Mold and other contaminants are on your coil inside your furnace. Cure is to have the coil cleaned professionally using chemicals to kill off the mold.

https://www.askmehelpdesk.com/heating-air-conditioning/furnace-installed-ac-circulates-sulphur-smell-99980.html