Synching your carbs by eye isn't accurate at all. I bought one of the carb tools from Jeff, and checked mine, and they were both off at idle, and on up in the RPM's. It doesn't take much of a difference between the air mixture screw adjustments, idle screw adjustments, and/or throttle cable adjustments to make one side a little richer than the other. The side that is rich will smoke more. If you're that worried about it, spend the money, buy the carb synch tool, and do it right. If your compression is good in both sides, and there isn't anything wrong with either exhaust, then it has to be carbs, or reeds.