It might just need to be used more and broken in. lots of carbs have stickey barels when they are new. Also you could try and packing the boot of the carb with gease. this will keep the boot sealed and keep the barrel of the carb lubed.

You can use another carb but .21 carbs have different diamater carb bases. OS, RB, Novarossi.. any high end carb would be good to use as a replacement if you decided to go that route but you will have to be sure they carb base diamater matches.