Mercury Mariner 200, 225 Optimax Outboards Service Manual, 90-855348, Page 207Get this manual
As the airfuel ratio becomes leaner, combustion temperatures increaseHigher combustion temperatures raise the NOx content of the exhaustBut, enrichening the airfuel ratio to decrease combustion temperatures and reduce NOx also increases HC and CO, as well as lowering fuel economySo the solution to controlling NOxas well as HC and CO is to keep the airfuel ratio as close to 14.7: as possible OUTBOARD HYDROCARBON