If I run water in the guest bath sink I get an oder of "old cigars". I have removed the pop-up assembly, plug the overflow hole, and filled the sink with 1/2 gal bleach and water to soak in overflow hole, as well as running the solution into the hole with a funnel just to make sure all bases covered. Also cleaned sink trap and have used stopped up drain solutions. No gunk anywhere, everything clean as a whistle.
However, nothing works.0

If I run water in the guest bath sink I get an oder of "old cigars". I have removed the pop-up assembly, plug the overflow hole, and filled the sink with 1/2 gal bleach and water to soak in overflow hole, as well as running the solution into the hole with a funnel just to make sure all bases covered. Also cleaned sink trap and have used stopped up drain solutions. No gunk anywhere, everything clean as a whistle.
However, nothing works.0

But you didn't get it all, Unless you stopped up the sink at the tailpiece, so all the cavities are filled, you haven't reached the source. (see image),
Try this, Remove the "J" bend from, the trap and block off the tailpiece. Now fill the sink to the rim with bleach and let it set over night. Flush with boiling water. Smell all gone? Good luck, Tom