An improperly adjusted gas furnace will cause carbon monoxide poisoning. (Wrong!)
Properly adjusted gas burners shown. Even when misadjusted, the carbon monoxide from the burners goes up the flue. However, if there is flame rollout, carbon monoxide will enter the interior space around the furnace and may eventually be pulled into the return air stream.
The only way a poorly adjusted gas burner in a residential furnace can cause carbon monoxide poisoning is if there’s a problem with the combustion air, the flue or the return-air system!
Even if the burners are covered with grease or rust to the point of having yellow-tipped, lazy flames (maximum CO formation), the poorly burning flame will still have its products of combustion float up the flue. However, if there is sufficient flame rollout because of a hole or crack in the heat exchanger, the products of combustion (including carbon monoxide) will be pushed out the front of the burner compartment on atmospheric burners. If that happens, there is a possibility of those fumes being pulled into the return air stream and then pushed throughout the home.
If there’s an odor or combustion gasses entering the home, there are OTHER problems besides misadjusted burners!