I just inherited a Weber on my 90 4.2. You need to go to RedLine.com, a Weber distributor and check out the site for instructions. IIRC the idle mix screw should be 2 full turns from closed to start with. Then you experiment screwing it out a bit to see when it starts running rough. If its rough to begin with, turn it back in up to 1 full turn to try and find the sweet spot where the truck is running OK. When I got mine, the IM screw was 4 full turns out, running so rich you couldnt stand the unburned gas smell out the back. I rolled it back to 2 turns out, then rolled it back 1 to 1 full turn out from closed and its running fine and no smell except the little oil burning. The site also says that if you have to turn it in/out more than 1 turn, you may need to buy different jets.
Be careful when you turn the IM screw closed, dont overtighten it or you will kill your carb.