bad flat spot, Help!

[clerky clerk]

Hi all, I have a new problem with my ec300, when nailing the bike it pulls up the rev range and hits a flat spot where the revs just hold and after a while it then pulls through and off it goes, its a bad flat spot. If im in a higher gear and drop downe a so the engine is reving high it misses the flat spot and im away. This does not cause any problems in the technical stuff as i dont need these sort of revs but if im out in to the open or going off at a start i need front wheel lifting acceleration but im not getting it. I had a full top end rebuild 10 hrs ago and the bike has foot breaking compression it had the reed spacer removed before i got it, the pv is working correctly the air filter is clean it has the N1ef needle on the second slot, the gear box oil is changed every other time its used, i have a iridium plug and use castrol mx fully synthetic oil at 40:1. Any ideas?

[cvl05]

How high in the revs and what throttle position is it when this comes on. It is likely a jetting or spark issue, 2t engines can be very picky about using the correct plug, years ago i did some playin around with plugs on a moded kx500 i was racing and found that all but the oem plug gave me poor performance in one or more areas so i always stick with the stock plug. Also the transition between pilot, needle and main jet may be too rich and causing it too bog or sputter so i'd look at the plug and see what it looks like. Check the float level in the carb as well. It's something simple likely