Welcome to FABO! Does it run fine once it's at normal operating temp? If so, it would be a choke problem. If it runs bad at all temps, it could be many things. I'm assuming that all the tune-up stuff has been done? Compression test, valve adjustment, points, plugs, etc? If that was all done and you've narrowed it down to carb, start with the basic adjustments. Adjust the choke so it closes all the way when it's cold. You bend the choke rod to do this. Run the mixture screw in all the way (just lightly seat it so you don't ruin the needle or seat) and back it out 2 turns. When it first fires up, the choke pull off should open the choke slightly so there is about a 1/8-1/4" opening to let air through. Bend the choke pull-off rod to achieve this measurement. If the choke pull off doesn't open the choke, it may have a bad diaphram and needs to be replaced. You can apply vacuum to it to test that. Once the engine is at full operating temp, readjust the mixture screw as necessary to get a smoooth idle. If it still won't work right, you'll have to rebuild or replace the carb.
