I agree with checking mixing screws. Mine ran EXACTLY like yours and come to find out, i had a crack in the carb where the fuel filter screwed in so it was leaking a tiny bit of fuel. Its a 84 2.8 that got 20-22 mpg!!! Im not sure exactly how but i think it was because i had the mixing screws about 3 or 4 turns out and the idle screw on the throttle closed almost completly(gives it that cammed idle sound!). Also it has no idle speed motor or whatever, no a.i.r. pump, straight exaust, my AIT sensor is plugged in but hanging over my heater hoses(custom air cleaner), all vac lines from solinoids on fender are pulled out but solinoids still hooked up to comp., no a/c, everything else is stock. But since i had to replace that carb cause it was cracked, im getting shit for mpg's and its hard to start in cold but i havent really tuned on it cause i havent had time and i have another truck i drive everyday for work.