Jon, here in the uk we don't mess with jets too much, think the gassers are pretty much spot on for jetting.
factory kev and shirty will sort this problem if it is one, but my first trip would be to the dealer.
However, assuming you know how the choke works and you have new clean fresh petrol and have a good spark then first check float level then the banjo filter. If not it may be the choke.