As the engine speed increases the idle becomes smoother and emissions fall. Setting the idle speed to 1200 RPM will usually produce a significant decrease in HC and CO levels, as well as as smoother idle and allow you to lean the mixture out more if you have installed larger sized injectors. This is covered below under the air mass sensor section. Air Mass Sensor - The air mass sensor is used by the computer to adjust fuel delivery at start up, idle and during sudden throttle changes. There are two adjustments to the sensor, both of which are normally sealed.
Idle mixture is controlled by a screw that looks like the idle speed screw.
On the side of the sensor there will be a port that looks similar to the idle speed adjustment port of the throttle body, except that a metal plug seals the top. Remove the metal plug by drilling a small hole in it and extracting it with a screw or punch. Turning the screw out will lean the idle mixture and turning it in will richen the idle mixture. The other adjustment requires removal of the plastic sensor cover. Carefully use a knife to cut out the silicone sealer around the outer edge of the cover and then pry it up with a small screwdriver.