I have been having a hell of a time with my truck. It was running rich so I changed the O2 sensor and it still smelled like it was rich so I took it in to be looked at. The mechanic adjusted the timing to specs (I think) and now it idles really low (about 600 rpms), It hesitates sometimes, and it still smells like its running rich. The truck just overall feels like the power isn't up to par. Any ideas on what I should check out?

Thanks