After completely stripping the carb body -- including the needle jet assembly from inside the float bowl [you need a tamper proof torx tool] I clamped mine to milling machine table and bored it out to approx 35.5mm. The stock 220 bore is offcenter from the carb body casting whereas your overbore will be centered, it's lower diameter coinciding with the original bore's OD. In other words all of the metal is removed towards the top of the carb or you screw up the relationship between the needle jet assemblyl and the bore. It now revs as hard as my KX 250. It doesn't appear to have hurt bottom end, but I also milled the head - and this improves bottom.