Nitro or glow engines use nitro fuel but it's actually a mixture of fuel and air that goes into the engine. The right fuel/air mixture keeps the engine running at its best. The wrong mixture can cause overheating, excessive wear, or cause the engine to stall. This fuel/air mixing takes place in the carburetor.
The high-end and low-end needles on the carburetor are adjusted to control this fuel/air mixture. To lean the mixture (add more air) make clockwise turns. To richen the mixture (add more fuel) make counter-clockwise turns.