I have left my bike alone with regards to carburation modifications but I have had the choppy feeling and it was down to the cush drive rubbers a couple of times and the chain slack a couple of others.
For reference though given your speeds V gears I have always ridden 30mph = 3rd gear 40mph = 4th gear With the nature of a big single it's going to be choppy otherwise if you're trying to hold a lower gear.