Check the fuel lines. Sounds like the fuel isn't being drawn until the carb is WOT... Not exactly a good thing. Most 2Cs do have a flat spot in the midrange, but nothing as bad as what you're seeing. You may want to play around a bit with your low end needle valve just to make sure you're not too lean down low. Also try a colder plug. I used to think that hot plugs were the thing for these ABC engines, but I've been experimenting with different plugs and have found that colder plugs actually work better with the higher nitro (I use either 15 or 20%) content. Power with my setup is extremely linear.