Sound like it's too rich on the High Speed Needle. Try to richen it a bit. I hope this information from Josh Cyrul helps you tune your engine:
Tuning Your Nitro Engine 101
Each day you arrive at a race track to race your nitro engine there are many variables that change: air temp, air pressure, humidity and even minor differences from one gallon of fuel to the next. These differences can really affect your engine's tune and overall performance. Here's my general routine that I go through to tune my engine to get the best starting point to work from given the variables for the day. This routine can also be used if you decided to change nitro content or even a pipe or manifold at the track.
To start, raise the idle by closing the idle needle about ¼ turn from where the engine ran last. This will keep the engine from flaming out when running through the routine. Next, it's to the starter box to fire up the engine. Make sure to “blip” the throttle a few times to warm the engine up to a reasonable temperature. Open the top end needle a full turn and then slowly open up the carburetor to full throttle. Once you are at full throttle continue to open the top end needle until it nearly flames out. If it does flame out just close the top end needle about ¼ turn so you can restart the engine and continue with the process. The idea behind this is that you are sending a large quantity of fuel through the carburetor with very open needles, thus flushing out any dirt, residue or after run products from the engine. From here, close the top end needle (carburetor still at full throttle) until the clutch starts to engage and the wheels will spin. We will come back to this needle to finish tuning the engine for racing.
Setting the bottom end correctly can not only improve throttle response and bottom end power but it can also increase the consistency and reliability of your engine. To set the bottom end you should listen to the idle speed of the engine. Pay attention if it speeds up or slows down over a 10-20 second time frame. If the idle speed increases then you should open the bottom end needle. On the contrary, if the speed decreases then you should close the bottom end needle. Once you have achieved an idle that stays constant for about 1 minute you can begin to open the idle needle, which will lower the engines idle speed. Open the needle until the engine flames out and then close the idle needle about 1/8 of a turn and restart the engine. Your engine should idle for 45+ seconds before the idle will slow enough to flame out. This should yield a very low and consistent idle speed that will give you better fuel mileage, lower engine temperature and better operation of your clutch.
After tuning the bottom end and idle we are now back to the top end. The top end is usually best set out on the race track, but here's a way to get it close on the bench. Open the carburetor to full throttle (never for more than 2-3 seconds before returning to idle) listening to achieve a nice, clean sound. Take your time with this, it will greatly affect the life of your engine!! Once the engine sounds good, you have to make sure it isn't too lean. For a long straight, hard braking and then hard acceleration to the next corner out on the race track this is the best way to simulate that cycle on the bench. Open the carburetor to full throttle for 2-3 seconds, close the carburetor for 2-3 seconds and then back to full throttle for 2-3 seconds. Your engine should rev as nice on the 2 nd blast as it did on the first. If the engine hesitates then it is too lean and you need to open the top end needle. Remember, it's always better to start a little too rich vs. too lean!!
This routine should get your engine ready for the track each day before you race. While you are driving your car around the track remember to listen to the engine. Watching the smoke from the engine's exhaust is ok, but a lot of times it doesn't always give you a consistent reading of your engines tune. Different fuel mixtures (different oils or oil percentages) don't always yield the same smoke so it's important to have another gauge such as throttle feel, sound, as well as a temperature probe are all good ways to gauge your engines performance. Be patient and have fun!!