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Nitro Engines - 101 For a nitro engine to run reliably and to produce maximum power, the carburetor must be adjusted correctly. The throttle body controls how much air enters the engine, and the needle valves meter the fuel. Most carbs have two needle-valve screws: the low -end and a high-end. Turning the adjustment screws in (clockwise) leans the mixture allowing less fuel. When turned out (counterclockwise), the mixture is richened and allows more fuel to flow through the carb. Fuel is constantly delivered to the engine by the low-end needle (idle). As rpm increases to 2,000 to 3,000, the midrange, additional fuel is drawn from the high-end needle. The higher the rpm, the more fuel is drawn from the high-end needle. A properly adjusted carb will deliver fuel evenly throughout the entire throttle range to produce a smooth throttle transition from idle to full power. Always adjust the low-end needle first. The low-end mixture should be set as lean as possible but not so lean that the engine doesn’t instantly transition from idle to the midrange. If the idle mixture is set too lean, the engine will hesitate and might quit when you advance the throttle quickly. If the idle mixture is set too rich, the engine will sputter and burble with excess fuel as you advance the throttle. You will know the mixture is correct when you advance the throttle quickly and the engine responds clearly and quickly. Set the high-end needle so that the engine will produce maximum rpm without overheating. If the high-end is set too lean, the engine will sag and slow down as it overheats. Don’t run your engine too lean, as this can damage it. If the high-end mixture is set too rich, the engine will run roughly and won’t develop full power. It is better to run rich than lean. Make small needle-valve adjustments – 1/16 turn at a time for the idle and 1/8 turn for the high end. After each new adjustment, allow the engine to run for a short time, and continue to lean the mixture until the rpm begin to drop. When this happens, back the needle off ¼ turn (rich). Check the idle setting again, and you’re done. If you do have to readjust the idle setting, readjust the high-end also, as both needles affect overall performance. Remember, factors such as temperature, humidity, air density, and barometric pressure effect engine tuning. There is not one simple setting for your climate. You may need to tune your engine several times per day.
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