Check the choke is winding off correctly - i ended up winding mine all the way out.
Also it's possible the carb is 'icing'. Unsure where you are, but if it's more prevalent at colder speeds - I found my secondary was icing shut and I'd have to shut it off a few minutes while she warmed back up.
Some fine sandpaper on the inside of the carb, the butterflies etc fixed mine right up.