Carb inserts are a tuning aid that limits/changes the amount of air that flows through your carb. Generally more air requires more fuel which creates more power. It's a balancing act between run time and power production. At WOT with a 9mm insert the engine should make more power than with a 6mm insert. I believe they also affect thermodynamics, how the fuel air mix flows, turbulent vs. laminar flow. Maybe someone else can chime in on the science, those days are far behind me...