Kaswasaki KX 125 1991 Losses power on high revs then cuts out?

I have a 1991 kawasaki KX 125 its had a new piston + rings the carb set, float set and also the needle. the timeing has also been set. i have brought a new racing spark plug wich produces a good strong spark. It starts ok but when riding it it pulls of then looses power its like when its suposed to hit the power band it gets bogged down and looses all power. the arm for the power band moves free and moves out then the bike is been reved in a staionary position. Can anyone Help.
Thanks Chris R (wolverhampton england)

chris roden
February 2009