Kolhler Command 18 v twin engine missfire?

Im rebuilding a friends husqvarna tractor into a race mower... first thing is first though.. The engine has a missfire on the left cylinder (from a behind the steering wheel perspective) I checked spark plugs they seem fine, I switched plugs to see if the missfire follows and it stayed left. spark plugs are out of the troubleshoot list.. I know little about this engine so what is next on the list of things to check in finding the cause of a missfire, and or sputtering of the left cylinder. thanks...