timing

is it possible to adjust the timing on a 98 burb? I was looking at my plugs and noticed that they had some carbon build up on it. In the chilton's manual it doesn't talk anything about adjusting the timing.

Non adjustible

So what could be causing the carbon on the plugs? The last plugs that I had in there looked great and had no carbon on them I have had these for about 7k and have noticed a drop in gas mileage and slow in get up and go. I'm using bosch plugs.

Go back to using the AC Delco platinums, not many plugs will beat them