Mercury Mariner 4, 5, and 6HP 4-Stroke Factory Service Manual, Page 102Get this manual
EMISSIONS
Stoichiometric (14.7:1) AirFuel Ratio
In the search to control pollutants and reduce exhaust emissions, engineers have discovered that they can be reduced effectively if gasoline engine operates at an airfuel ratio of 14.7:1The technical term for this ideal ratio is stoichiometricAn airfuel ratio of 14.7:1 provides the best control of all three elements in the exhaust under almost all conditionsThe HC and CO content of the exhaust gas is influenced significantly by the airfuel ratioAt an airfuel ratio leaner than 14.7:1, HC and CO levels are low, but with ratio richer than 14.7:1 they rise rapidlyIt would seem that controlling HC and CO by themselves might not be such difficult task; the airfuel ratio only needs to be kept leaner than 14.7:1However, there is also NOx to considerAs 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 COis to keep the airfuel ratio as close to 14.7:1 as possible