had the same problem with mine, its normaly the idle air value, (looks like a heart with loads of tubes going in it) its a 2 min job take it off and spray with some carb cleaner in side.
Also if the bike has been stood for a few years it would be wise to take the plugs out and change and you'll find that number 1 or 3 cylinder plug will be oil fouled !!!
Its norm for these beasties, and as a bike they dont like going slow so tend to be lumby.
hope this helps