Re: very bad gas mileage.
Well, a couple things come to mind, as I also run that carb.
Are your wheel bearings free?? Bad bearings no matter where they are will cause your engine to work harder for the same speed achieved by good bearings, hence using more gas.
Does your bike pedal easily when clutch pulled? Possible engine bad engine bearings.
Have you tuned the carb? The carbs come stock with a setting on the needle in the middle.
What speeds are you doing?? Run to fast or too slow, your using more gas than you need to. Each engine has its own sweet spot according to how it's set up.
Hills?? Use more gas goin up hills.
Only a mile?? Engine doesn't really warm up till 4-5 mile mark, then fuel savings kick in.