Most likely the slow jets in the carbs are plugged. It is VERY difficult to get the float bowls off and the jets out with the carbs in place. Some will claim its possible, but in the long run its probably easier to pull the carbs off the bike, get them on a bench and give them a good cleaning.
If they are very lightly plugged, you might have success draining the float bowls and re-filling with Seafoam carb cleaner, then letting them soak for several days.
Kevin