My opinion to you problem is a rich mixture, you may need some help to tune it or if you could tie it down that would be good. At full throttle lean out the high end till it cuts back on rpm.
When it does richin just until you regain that rpm not anymore. Go to idle and check your transition if it spits or give a fourstrock sound especially at half throttle you will need to lean out the low end, keep doing that until you reach your disired performance, carefull not to lean the low end too much cause it will burn up.

Good luck.