You can try tinkering with the mixture on the carb. I had my Carter carb replaced a long time ago (with what, I actually have no idea -- that's a long story) and my mileage dropped from 16 or 17 mpg to 5. The garage (a Jeep dealership, no less) refused to do anything about it. Skipping a lot of the detail here, I finally got a refund for the work they did and then adjusted it myself. Mileage went right back up where it had been, and has more or less stayed there since.
'81 CJ-7, 175,000 miles
Stock 258, SR-4