You need to change the main jets inside the carburettor, these are numbered to indicate the amount of fuel they pass compared to one another.... this applies to 99% of jap bikes... and there is no hard and fast answer, it is a question of guesstimation and then experimentation, riding it and pulling the plugs to inspect them.
HTH etc

thanks john
       dropped jet size by one size front and back but will possibly need to go one size smaller at moment,124 rear and 122 fwd(original 126 rear 124 fwd.goes well with less popping when decelerating.but finding it hard to get a 120 yamaha australia dont have them any more.
           andy