you can adjust it on the metering unit, i think you disconnect the mutiplug on the black box attached to the meteing unit at the back of it ( can't think what it is called, potentiometer maybe? ) and then remove the small grub screw that is in front of the plate ( with air filter removed ) you then use a small hex key to adjust the co , be warned though you will never get it spot on by guessing and the unit is easily damaged so don't rev the car whilst adjusting and then get it straight to a tuner.