I just serviced mine, at the same time refitted the Remus Revolution can with baffle, and did the valve clearances, and also did the Steptoe ccp mod. Because I did all these things at once I don't know what made it better, but I now have a much better transition from neutral throttle. Used to feel like an on/off switch. Also it doesn't backfire anymore (could have just been an air leak).
Not much help that really! But mine doesn't surge anymore with those changes.