www.mikuni.com/pdf/vmmanual
there is a graph in this manual that will explain the size difference. you can get the right air fuel mix but will lower your output if too small. might not be a significant difference with the 400 though since it is lower on the output scale anyway. just my 2c
side note, I'm having trouble with the exact link. Google mikuni VM manual and it should pop up if it doesn't work.
