Is the engine fully warmed up and only operating on 1/2 choke? The engine still might be a little cold. My engine runs like a turd when its cold (0* and below) until it warms up.

Otherwise it could be a mixture adjustment. Are you sure your float is adjusted right? Mixture screws adjusted right? Power valve ruptured?

How much vacuum are you getting at idle?

Also, what color are your plugs?