With any carb changes they need to be tuned to the bike they are being fitted too, getting the jets, needle position etc, right for the conditions as well the bike all plays a part in tuning, unless the carb has been built for your bike but they still need fiddling and adjustments made, putting a new carb on and not touch it would be luck at its best and of course start with new spark plugs, make sure timing is set right and valves adjusted, they all play a part to tuning.
If your old carb is not idling right then something is not right and might be just something simple to get it idling.