The later models of Yamaha (1982 & newer) use a YICS ( Yamaha Induction Control System) which is a vacuum balancing system in the intake manifold. The YICS system helps the cold start back fire that the non-Yics machines are prone to. To balance the carbs and set the idle mixtures properly the YICS must be rendered deactive so a true vacuum can be established. A special tool is required to seal the YICS passages while this adjusting is going on.