Mercury Mariner V-250 V-275 Outboard Service Shop Manual 1990, Page 82Get this manual
Idle Jet The idle jet meters fuel-air mixtureIf jet with small orifice is installed the id le fuelai ratio becomes leanerIf jet with larger orifice is installed the idle fuelair ratio becomes richer
Vent Jet
The vent jet su pplies less than atmospheric pressure to the fuel bowl at mid-range, wh ich results in impro ved fuel economyIf jet with smaller orifice is installed the mid-range fuelai ratio will be leanerIf jet with larger ori fice is insta lled (or jet is removedthe midrange fuell air rati will be ri cher