Most important part of dialing in the Weber 34ICH is to make sure the ignition is setup rock solid correctly before touching any adjustment on the carb. Make sure the points are gapped correctly, the timing is spot on, and all ignition components are in good condition. If ya try and adjust the carb with the ignition off it'll be pointless because it'll never be right no matter how much you fiddle with it.
The only adjustment you can do is the idle by the idle mixture screw and of course alter the rpm of idle with the throttle adjustment screw. Beyond that you'll need to replace jets to alter how the carb runs after idle.
I've gotten 17 mpg with the Weber 34 ICH.
Walker
1968 Series IIA-"Ronnie"
88" SW, 2.25L Petrol, LHD