Check out this article on the US site on the mid range flat spot
http://www.sohc4.net/?p=275I found raising the needles (lower clip one notch) helped allot but it still wasn't quite right so I was going to try further jetting changes, but after changing the filter it's miles better.
What I find a bit odd is I had previously tried it with and without a filter and it made no noticable difference to the flat spot, but I'm not certain of the order I tried it in so it may have been before raising the needles.
I didn't try the suggested ignition changes as I have a Boyer kit fitted.