I need to know how to set the distrubtor - 1998 GMC Jimmy

The ignition timing is not adjusted with a timing light or with the engine running, and to set the ignition timing follow these procedures.

There is a mark or notch on the distributor housing that the rotor should be pointing to when the engine is on TDC. (Top Dead Center) This "Static" timing is all that matters and the computer will be able to control the timing as long as the ignition rotor is in that position when the engine is at TDC.

1. Place the engine at top dead center.

2. Look under the distributor cap and find where the number one terminal runs under the distributor cap, and where that position on the distributor cap corresponds with the distributor housing, and it should match up to a mark or a notch on the distributor housing (usually has a #6 for 6 cylinder engines or a #8 for 8 cylinder engines) indicating the number one TDC alignment position.

3. With the engine on top dead center the ignition rotor should be pointing to the number one TDC alignment mark or notch that is on the distributor housing, if it is not then loosen up the distributor and turn the distributor until the ignition rotor is pointing to and aligned with the TDC alignment mark or notch on the distributor housing and then tighten down the distributor, the engine should now be "Static" timed. (The more precise that you are aligning the TDC alignment mark with the ignition rotor the better the engine will run, and it will be less likely that there will be a camshaft to crankshaft correlation problem)

If the distributor can not be turned enough to align the ignition rotor with the number one TDC alignment mark on the distributor housing, or the distributor does not set properly and will not allow the installation of the spark plug wires then the distributor is not installed correctly and is most likely a tooth off and it will need to be re-installed correctly. (The distributor should set like it is shown in the firing order diagram when it is properly installed)

Here is a firing order diagram to also help assist you.

Some distributors have the hold down bracket secured onto the distributor housing and the distributor can not be turned into alignment and then tightened down into that position, and these distributors will only fit into the engine and correctly align on "Static" timing one certain way, and when it is correct the distributor hold down bracket bolt can be installed and the TDC alignment mark on the distributor housing will remain properly aligned with the ignition rotor.