Have you changed anything, or did something change itself?
As Gort says, learn to read the color of the spark plug.
http://www.verrill.com/moto/sellingguide/sparkplugs/plugcolorchart.htmWith your "stage 1 kit" you can try turning the idle adjustment screw in (clockwise) as far as possible without hurting engine performance. You can try lifting the jet needle less by adding a 3mm washer (about .5mm thick) or two on top of the thin kit spacer, again as much as possible without hurting performance.
Unlike a post-'86 car with a mass airflow sensor and other electronic controls, our motorcycle fuel system is primitive. A dirty air filter element will cause rich operation. If your K&N element looks like a lint blanket, it needs cleaning & re-oiling. A $15 HiFlo brand replacement from a non-Suzuki shop is a good choice if needed for those with a dirty stock air filter.
If you changed anything, retrace your steps; maybe you did something that caused the problem.