Runs at FAST idle, not slow

i agree with Bill 100% so just to reiterate, that whoever set up your carb didnt do a good job. it sounds like the float level is set too high and flooding out or the choke is out of wack. nows the time to learn how to rebuild those carbs. they are just about the easiest carbs in the world to rebuild and get set-up. there are very few parts to them. i would say it is definately a carb issue. if the mixture screw isnt doing anything, then the idle circuit is clogged or there is way too much fuel getting into it.