Remember that lean misfires can cause rich-smelling exhaust. The fuel vapor comes through and the smell even though it's lean. It would be easier the know what's going on if you had access to a four gas analyzer and a scope. I'm betting you don't have either of those laying around so for now I would replace any worn secondary ignition parts (plugs, wires, cap, rotor) and then clean your fuel injectors. You probably don't have the tool for that either, but if you ask around, someone in your area might. Or you could pay a shop to do it. Independant shops are usually less expensive for that kind of thing, but shop around. It really does help (I used to think it was a big rip-off until I started performing the services and testing the results before and after (required for GM warranty)).

Also, if you have an exhaust leak BEFORE the O2 sensor it can really screw up the way it runs. The sensor sees the extra oxygen and fattens up the system even though it isn't needed.

Hope that helps,

Clay