1995 GMC Suburban how to adjust timing

I have asked a similar question, which may be helpful... my question is: "does this gmc have EST - electronic spark timing?" If it does, then you must unplug the EST in the relay box on the firewall before you use a timing light to view the "teeth" and the timing mark on the pulley, using a strobe light. Best done by a mechanic, amateurs are often injured by fan blades and belts.