Some typical places for air leaks (two strokes)--backplate, carb throat, fuel tubing.
Do you have bubbles in the fuel line? If not, then you can eliminate everything up to the carb. Try loosening the carb set screw, and re-seat the carb into the throat.
You said you checked the backplate.
There could be an air leak without visible signs of "blowby."
It's weird that you only get it at high speed, though. It idles good? Usually an air leak will keep you from getting the needles dialed in on both low and high ends. You are sure it's not just out of tune? Is the engine brand new? Was it broke in properly? Might just need to richen both needles up a bit.