I stuck a weber 34 on mine and haven't been very happy. It's got the electric choke which I still haven't figured out. As far as gas milage, I haven't got over 23 yet. Still working on it. I never had a problem with all the emission crap but I suppose it could have been a problem at some point. As well, the computer thing behind the glove box has something to do with controlling how everything comes together.
It's all complex, but I guess that's why the hitachi got such good milage when tuned right. Mine was dumping gas or I would have stuck with it. I saw a weber on ebay that bragged it would get better fuel milage and more h.p. then the stock. Maybe h.p. but definetely not on milage.
If I could do it all over again, I would have maybe rebuilt the hitachi, and made sure all the hoses were correct and snug. Then I would have checked or tested the remaing components if possible and replaced what I could with what ever was affordable. Of course, if I put the hitachi back on and it performed the way I wanted it too, I could assume that the other components were functioning properly. The diagnosis on all that crap could be intimidating.
I spent around 400. for my complete kit. I could have rebuilt the stock a bit cheaper and put all that money into a header and other exaust components and probably got better results. I also found that there are fewer and fewer shops that work on carbs, and the ones in my area don't want to touch a weber, only a carter or holly if at all.