Replacement of the carb is not necessary.

The Rossi carbs installed on many .40, .45, and.53 had too long of a spray bar. This would not allow proper adjustment of the low speed mixture screw. The problem you described is typical of these carbs.

To fix it is simple. Pull the carb from the engine. Remove the high speed needle and throttle barrel. Using a drill bit that is slightly larger than the spray bar, mill the end of the spray bar until you have removed 1/8"-3/16" of length. The brass is so soft that you can twist the bit with your fingers and prevent accidental excess material removal. Clean it all up, reassemble, and remount it on the engine, remembering to press the carb firmly into the bore while tightening the pinch bar.

You will find that you can now tune your engine to idle as normal.

Have Fun!