Tuning the Needle Valves
Nitro engines use needle valves, or “needles” for short, to control how much fuel is in the fuel/air mixture. Some sport engines only have one needle, but most have two: a “low-end” needle that controls the fuel/ air mixture from idle to about half-throttle, and a “high-end” needle that controls the mixture from about half-throttle to full throttle. The needles are tuned by turning them with a screwdriver. Turning the needle counterclockwise will increase the amount of fuel in the mixture, or “richen” it. Turning the needle clockwise reduces the amount of fuel in the mixture, or “lean it out.”
When tuning the needles, think of the hours on a clock face. Once your engine’s tune is close, you should only need to make adjustments “one hour” at a time. Unless your engine is way out of tune, adjustments more than “three hours” (one quarter turn) should not be required.