97 GMC Yukon: How to adjust the timing

[quote="backyardmechanic"]Your cars PCM controls the timing for ignition and injectors. There is no adjustment required or allowed. Why do you need to adjust the timing. Is there something wrong that I can help with
Backyardmechanic :D[/quote

We initially took it to a shop to get the fuel pump repaired. We drove it in but had to tow it out. It wouldn't turn over at all. We replaced the fuel pump pressure regulaor, replaced all 4 O2 sensors, throttle positioning sensor, idle control sensor and cleaned the spark plugs. It turns over but seems to not be getting any fire. We were told that the timing was off. Could it be something else?