You could have a bad plug. Try an O.S. #8. That's a good one for 15% nitro fuel. You could also have a rather rich midrange. Likely, the idle mixture is a bit too rich.

To set a carb, you need to start out with the idle mixture too rich. Open the high-speed needle 2-3 turns out from closed. Start the engine at 1/4-1/3 throttle, and open to full once it starts. Lean the high-speed mixture a bit to get the engine at a higher RPM, and then remove the starter battery. Lean the high-speed mixture to just rich or peak RPM. If the engine will run out a full tank of fuel with little mixture change, you can set up the idle. Retard the throttle to about 1/2 and adjust the idle mixture a bit. Retard and adjust some more, a bit at a time, until you get a good idle. You should be able to idle at 2,500 RPM or below.

If the engine hesitates and then picks up when you gradually open the throttle, the idle is likely too rich. Lean it a bit and then change the idle RPM so that you have a good idle. Remember, when you change the idle mixture, you'll have to change the idle RPM. Many times, you can get a very good idle at a very low RPM, but you'll have a lousy throttle transition. You'll have to trade some RPM to get a better transition.