You're getting 28 mpg. There's no reason you shouldn't be able to get 40-50 mpg. Time to start re-jetting. Either do it yourself or have a shop do it for you.
My advice is for you to find a good independent shop and go from there. Also do some research (online) and find out what makes your Keihin carb tick. Then you'll be able to talk carb with your mechanic and understand what he's talking about.
This stuff isn't that difficult and you could easily do it yourself with some research and some trial and error.