Timing 92' Geo Metro 1L

I recently had a smog test on my 92 Geo Metro and passed everything except the HC at idle. This reading was extremely high. The HC at high rpm was fairly low and easily passed. The question I have is this: What would cause the car to pass at high rpm but not pass at low rpm? I have replaced the cap,rotor,wires,plugs,pcv valve, and now the mechanic at the station I took my car to says it probably needs to have the timing looked at. I would like to do this myself if possible but am unsure of the process. Any help would be greatly appreciated.

Well, I was able to do the timing on the car and it brought down the HC signifigantly, enough to pass anyway. They are still pretty high but now I can register my car. Timing the car to the manufacturer's specifications made the car run terrible. I am tempted, now that I passed the emissions test, to turn the timing back to where it used to be (I marked it before I started moving it) as the car ran great at where it was. Is there any disadvantage to doing this? Will I get better gas mileage if I leave it timed where it is or should I turn it back to where it was before I started this whole mess?