Hi the one thing you didnt mention replacing was the fuel tap seals. I had mine done and the problem was solved, The fuel system operates under vacuum and if the tap seals are duff it draws air in to the fuel system causing weak mixture and very hesistant response in the lower rev range, you can buy the repair kit from Wemoto, Carb kit( American site) or Gear4Bikes looking at about £12 or so. One way to test is turn the tap to prime and then back to normal position if it weeps a bit of petrol the seals need changing. It might not be the answer but its worth a try as it wont break the bank to do it.