Try changing the needles to Gossie carb needles from Holland http://www.ural.eu/en/cont.html
My mileage went from 25ish to consistently over 30. Sometimes I see 33mpg.
I also drilled the slides and messed with the jetting a bit. All very easy to do. I am currently running 125 main jets and 42 low speed jets with a stock engine other than Dunstall mufflers and a K & N air filter in the stock airbox.
I make a point to not run my Ural wide open. I can cruise the freeways at 60ish with a lot less than full throttle.
No engine is designed to run full throttle for long periods of time. Even commercial diesels with a 100% duty cycle achieve that rating at somewhat less than full throttle (often 85% of max rpm's with throttle to spare). If you need to run 70mph with it maybe you should consider swapping in an Airhead R100 engine (but watch out for final drive problems with the extra power).