I have heard others complain, but I thought the Rossi carbs about average on the ease of tune scale. Unfortunately, they use a very large carb neck. I don't think any other carb will fit without machining an adapter.
Many times tuning problems are the result of a poorly sealed tank lid, tiny hole in a fuel line, bad o-ring on a carb needle, etc.
If he can make over to my shop, I'd be glad to give it a shot. (Portage, IN)