I had the same problem with my 1990. What I did was remove the carbs and clean them. I cleaned and gapped the plugs then balanced the carbs. The bike runs MUCH sweeter now and the mileage has improved. If you feel like trying all that yourself be sure you have a Honda Shop Manual to use for a guide. I have heard that the Honda Manual is really the best and only one to use. The Manual is available online or you can order one from a Dealer. If you don't want to tackle it yourself then take it to a Honda Dealer.
Also, your post states that you get poor mileage but what you describe is the bike runs out of gas/quits when the gauge shows half a tank of gas is left. Is it really getting poor mileage or is it quiting with half a tank of gas still in the bike? Two different problems. If it is poor mileage then the carb work is probably in order but if its quiting with half fuel left then something else is wrong.
When the bike "runs out" of fuel,how much is it taking to fill it back up? How does this bike run? Any smoke of any color,some or all the time? My thinking is if this bike is getting app.18mpg,then some spark plug fouling should probably be occuring,or at least just not running well,low power etc. On previous threads,many of us discussed the poor calibration of the 1500's fuel sending unit. Thinking this one is just waaay off. Hope this helps the process.