If you don't have to have an emissions inspection, and are willing to spend the money, I undertand that the Weber carburetors are good upgrades.

If you have to pass an emissions inspection, you may be stuck with rebuilding your existing one or replacing with another rebuilt Hitachi. The following is a good link for rebuilding Hitachi carburetors if you are interested.
http://dimequarterly.tierranet.com/articles/tech_carb_tuning_guide.html