I don't think there are any physical differences between the various IDs of the BST33 - I think they were marked to make it easier to ensure the right carb for a given market was installed at the factory. If my assumption is correct, the only difference is jetting. I did a little looking myself and from what I can tell the jetting you have is probably the leanest of any of the DRs....stock main jets go as high as 132.5 for other markets, and correspondingly richer pilot/air/etc settings. Which is probably why that model is such a gas sipper..... I think some experimentation is in order - you can clearly go richer on the jet and the needle even with everything else stock, but as others have said you might not be happy with the results.....personally, on my canadian DR I went with the airbox mod and richened everything up for better response in the woods, but I wasn't worried about outright mileage. I'm happy, other than with a bit of surging on pavement at steady throttle....that I am told requires a pumper to really correct.