When I had my first ETZ, I originally had lots of problems with the carb that even now I don't understand. Initially it wouldn't move under 3k rpm, and various rebuilding of it seemed to give different results. One such example was after putting it together it was running so rich it didn't manage to go around the block before fouling the plug. I rebuilt it again and I got it back to the running as before. I recall that I felt that that was the best it was going to be... Until one day I crashed and the bike went down the road on its right side 20 yards.. but something had changed because as soon as I got on it and rode away it's lower (<3k) torque was suddenly there and since then it was a lot easier to ride at lower revs..
What I'm saying is that all you can do sometimes is just keep rebuilding the carb.
Other things you could think of is the spark plug a cold grade? Are you sure the air filter blocked isn't blocked? See what happens when you run without it? I'm not sure if you've said that you've changed the sparkie? I mean, not only have I fixed similar issues with using a hotter (b7 NGK as opposed to b8) but I have had a lot of issues with spark plugs overall that have lasted a very short time. If you were close I would come and have a look
Definitely double check the choke rubber. Good luck!