15% does not sound not too high, and a 9x6 prop should not cause over heating with a needle out so much. It sounds like a fuel draw, or air leakage problem. Here are some questions, and some things to check:
1) Is this a new engine or was it working ok and now it's not? If it was ok, what happened just before it started running bad? As you screw the needle out, does the engine show any signs of getting rich or does it stay at a high speed? Watch the fuel line closely while the engine is running to see if there are any air bubbles.
2) When it stops is it hot, or relatively cool? If it's hot, that's a sign that it's too lean. If it's not hot, it must be another problem.
3) As has been suggested, make sure the pressure fitting on the muffler is clear (you have connected the muffler pressure to the tank, right?), put a piece of tubing on it and confirm it's easy to blow air through it. Do the same with the tank, attach a fuel line and blow through it to confirm there are no obstructions. Also try plugging one of the tank outlets and blow through it to confirm the tank can hold pressure.
4) Attach a piece of fuel line to the carb and screw the needle all the way in. While blowing through the fuel line, screw the needle out. As the needle gets further out, it should be easier to blow through the fuel line. If it doesn't you may have an obstruction in the needle assembly.
5) Ten seconds is enough time for the fuel to start foaming. Confirm that the tank is well padded.
6) Check the fuel lines for holes, and confirm that the clunk has not been pushed to the front of the tank.
7) Confirm that the carb is firmly seated on the o-ring.
8) Sometimes air can leak past the threads of the needle, making it difficult to make the engine rich. Cut a piece of fuel line short enough that you can slip it over the threads of the needle assembly. Unscrew the needle, put the fuel tubing on and screw the needle back on.