You can use the Rossi carb restrictors on the V-Spec. I checked the diameter and length and they came out to be the same as the OS ones. I think that the RB ones are the same too. The smallest I have found is a 6mm from rossi. I run a 8mm at my altitude (almost 5280 ft.) and will run a smaller one when I run at a lower altitude. My theory is that as you go up in altitude a larger restrictor is needed because the air is thinner and should allow the power band to feel close to a lower altitude without running too lean. I have even went as far as running without a restrictor to have the full 9mm out of the carb. It worked ok, but choose to put the 8mm back in. I have be wrong in my thinking, but it seems to work so far.