Sounds pretty typical of a Banshee. They aren't known for low end. You can do some things to them to help keep it in the powerband at lower RPMs. 2:1 carb, FMF Gnarly/PT Mid/Dynoport 2:1 pipes, drop a tooth on the front sprocket, etc. One thing you might check is the setting on the needle. If it isn't set just right, it will cause a bogging before the powerband hits. What pilot are you running also?