Tuning Engine for Power
By now, your engine should be broken in by now. You can start to adjust it for maximum performance. 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 look like the picture. 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. 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

Tuning the High-Speed Mixture
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 "spit" test to check the temperature of your engine. Remember, you want the engine to run around 270' or spit to sit on the head for around 5-7 seconds.

Tuning the Low-Speed Mixture
The low speed mixture affects how the engine will perform at idle and low R.P.M.s. Like the high-end mixture, the "leaner" you run it the more performance you will get out of it. If it is set too "lean" it will stutter and even stall out. To properly set the low-end mixture, do the following procedures: Get the engine up to running operation temperature. Then from a complete stop pull full throttle, if the engine bogs down, you need to "lean" it out a bit. If it speeds up then bogs down, it's too "lean", "richen it up a bit. You want it to accelerate well with out stalling.