19mm gives better all round mid-range power and the 21 gives all out top range power but is useless for town riding and mid range power.
i would go for the runner 125 carb i was gonna but a 21mm delorto ds blackedition came up for sale cheaper so i got it
also in theory youd get better fuel economy with a 19mm (as apposed to the 21 but worse fuel economy than standard carb) sam :nerd:
yeah it's a better performance carb, a little more sensitive to jetting i beleive, keep an eye on it.