Hi mate,

I had the exact same problem yesterday, and I had to pull the carbs off to figure out the problem.

You have to adjust the linkage between the two carbs so the butterflys are open equal amounts at all revs. The rod that runs through the carbs can bend over time, meaning that one carb can be slightly off. I shone a torch through the carb as if throttle was on idle, and adjusted the linkage so that the amount of light shining through past the butterflys was equal on all carbs. There may be other wasy of doing it, but it solved my problem.

Hope this helps.