Cub: Go in the direction of DarZeelon, I have a lot of Rossi for years and from different manufacturing date and all of them have same problem. Mine runs max 5%nitro and one shim. Rossi carbs are smaller venturi making transition problem due porting configuration.You have tro options :

1-Go to http://shop.vendio.com/rossienginesusa and contact Sahak, they have Perry carb and will help you a lot if you don't know how to deal with Rossi engines.Just swap carb.
2-Bore your stock carb at least 1mm. Ideal will be 2mm. This is the difficult way and also need some special tools.

Before doing any mods, please add shims or lower your nitro to no nitro but remember, transition never will be same as an OS but they develop a lot of power.

David P.