You may have tuned yourself right past the lean point. A rich low end and overly lean high end will still produce good results at mid throttle.

Bring the engine up to where it runs well, then richen the high end slightly. Advance the throttle a little more and see how far you can take it. Then repeat the process. Richen, then advance the throttle again.

You want to find the point at which you can run wide open throttle, with slightly less then maximum rpm.

Now that the high end is set, start to work the throttle closed a little at a time. At some point of closing the throttle you may hear the engine gurgle. Continue to lean out the low end needle till you obtain a good idle with the carb open about 1/8".

Now check the high end again. You should be very close to perfect, but you may have to adjust with a click or two richer on the high end after adjusting the low end.

Always adjust high end first. Then move to low end.

I agree with blw, use the A3 plug, and use 10% nitro.