The first thing you should check is to make sure that when you pull full throttle the carburetor is fully opening. If you take the air filter off and pull the trigger all the way back it should open completely. You should always keep the engine within its intended operating temperature. Going above these could and most likely will cause damage. As you lean the engine out, it will run faster and faster till you hit a point where it will overheat. When this has happened it will start to stutter, hesitate, or even stall. The engine will over heat very quickly when the mixture has been set too lean. Check the engine often when leaning it out to make sure its not overheating. 

The simplest way to test for over heating is to put a drop of water or spite on top of the engine head. If it boils away instantly shut the engine down and let it cool off. If it takes 5-7 seconds for the water to evaporate away, then the engine is running at a good temperature. 

When you have the mixture set correctly you will hear the engine running smoothly and have a strong-sounding high pitch when you let it wind out. Running the engine a little "rich" is always a lot better then running it to "lean".

Once again: Lean = less fuel
Rich = more fuel

The high-speed mixture will affect the way engine runs at mid and high R.P.M.s. This is the main needle that you will adjust the most. Once you get you engine running good, this will be the only needle that you should have to mess with. 

Run the car on a smooth flat surface with enough room to let the model get up to top speed. Keep track of the speed as you slowly (1/8 of a turn at a time) lean the engine. You can lean it as long as you continue to have thick blue smoke coming out of the pipe. If the engine gets up to top speed and looses power most likely you have "leaned" it too much. You want the high-speed mixture to be lean enough to get good power and still keep the engine cool. Use a temperature gun or the water test to check the temperature of your engine. Remember, you want the engine to run around 270' or water to sit on the head for around 5-7 seconds.