1998 arctic cat 500 starts and runs fine for 20 minutes then idles very fast with slight backfire. Had this symptom the first six times but now the last two times it runs fine for 20 minutes then just runs rough and stalls. If you try to keep it running with some throttle it backfires bad. I have sprayed carb cleaner around engine and carb with no effect. Could it be electrical? stator? cdi box?