Mercury Mariner 4, 5, and 6HP 4-Stroke Factory Service Manual, Page 104Get this manual
EMISSIONS
Stratified Charge
A stratified charge engine only pulls air through the transfer systemThe fuel required for combustion is forced into the cylinder through an injector placed in the top of the cylinder (head)The injector sprays an airfuel mixture in the form of cloud into the cylinderSurrounding this cloud is air supplied by the transfer systemAs the cloud is ignited and burns, the surrounding air provides almost complete combustion before the exhaust port opensA stratified charge engine concentrates rich mixture in the vicinity of the spark plug (airfuel ratio less than 14.7:1)Elsewhere, the mixture is very lean or is comprised of air only