I've got a Cognito Moto airbox on my CB550 and my plugs are burning too rich. To get the revs down at idle I've have to turn the airscrew right in. Just one turn out to be precise. The bike runs and rides lovely but after a couple of hours the plugs are covered in soot. If I turn the air screw out to try and lean it out the revs climb. The manual idle screw is right out so I can't adjust that any more. If I reduce the jet sizes will this help? I'm thinking less fuel will reduce revs and lean the mixture at the same time.
