pwk28mm

unless you have a right bend pipe, or a toomey. i suggest you stay with the stock carb.

a 28mm gives you no gain over stock unless your stock one is messed up and non-working
 
what type of riding? tight trails? open feilds? etc... Only asking because a stock carb might be best. although depending on your riding you "could " benefit from a larger carb but a stock 26mm is a good carb for allot of variances. Also a stock carb can be bored slightly larger if need be and you get to keep the oil injection :) ...