Yip the backfire is excess fuel, Pull the carb,, check its jetted to manual spec,,Carb-mikuni VM34. Mainjet 190 -pilot-80. check the reeds too , there old now ,,pull the reed block and inspect,
The way you ride it Kristian, youy"ve killed 2 top ends, and the big bore kit will not be as robust as factory spec, i would buy a bigger bike, as then it will not be flat out most of its life,,,