timing and exhaust question

Could timing cause my suburban to idle rough? If so how do I correct the timing with the little timing gun. I have so far changed my spark plugs, dist. cap, rotor button, cat. converter. The problem is in my exhaust system some where I just can't find where. Also I looked at my plugs wires after changing the cap and button and saw black dust, and little white falkes of some kind. Really don't think thats supposed to be their or is it?


If you can't tell this is really my first time working on my chevy. :D