The carbon buildup the one cylinder has,is enough all by itself to cause you'r engine to knock.The carbon has probably built up enough that it has both raised the compression too high in that cylinder.As well as the carbon itself causing hot spots in the cylinder causing preignition.Tom recomends injector cleaner to try cleaning out the deposits.I've have my best luck cleaning up both fuel systems and carbon build up using Sea Foam.Either way,it's a good place to start.