The timing is not adjustable. See this excerpt.

 The Distributor Ignition (DI) system consists of the distributor, hall effect switch (camshaft position sensor), ignition coil, secondary wires, spark plugs, knock sensor and the crankshaft position sensor. The system is controlled by the Vehicle Control Module (VCM). The VCM, using information from various engine sensors, controls the spark timing, dwell, and the firing of the ignition coil. It is used on some 4.3L models.

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What you need to do is look it over real good for vacuum leaks. Then remove the EGR valve and inspect the pintle to see if any carbon is caught in it. After all that clean the throttle body. Hold the throttle wide open and scrub the backside of the throttle plate and surrounding housing with an old tooth brush and some carb cleaner. That should get your idle straightened out.