lets say it's at least an FLH, or a Andrews A. If you don't want to spent a ton of money, but want easy starts and reliability and bolt on Harley parts. Get a Bendix. They run good, get good gas mileage, easy to repair.
Another thought, but a little more work is the CV. If you don't mind spending a little time playing with a few things, this carb is great on Pans and Shovels. I've never seen one on a Knuck, but I've heard they work on them to.
There is a guy on E-bay that sells a solid mount adapter for the CV, so you can get rid of the rubber manifold.
One of the things I would do, what ever carb you end up with is get an air filter. Dirt is the killer to any carb.
Good luck, Pooka