The most common way to power an RC model airplane is with a 2-Stroke engine. These engines are easy to maintain, have relatively few parts, and can be tuned in just a few minutes.
An RC model airplane engine that is not correctly tuned can turn your favorite RC aircraft into a pile of toothpicks very quickly. Knowing how to adjust the needle valves on your engine is an easy task that gets easier with experience.
Standard 2-stoke RC model airplane engine carburetors have two valves on them. These valves regulate the mixture of fuel and air that flow into the engine. If the valves are adjusted properly, they will allow just enough air and fuel for the engine to run at peak performance.
If they’re not set right, too much or too little fuel will make it run either ‘rich’ or ‘lean’ which means that the engine might sputter or overheat and die. Then you get to learn how to make a deadstick landing, the technical term for landing (and often crashing) with no power. Your engine needs to be tuned to run right.
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High-speed needle valve
Look on the outside of the carb for a needle valve with a long adjustable handle. The first one is called the high-speed needle valve (HSNV). This one controls the fuel mix at full throttle and when the engine is running at top RPM. The second valve is usually found inside the throttle arm assembly and is the low-speed needle valve (LSNV). This one regulates how the fuel and air are mixed when your engine is idling or accelerating from a low speed to high speed.
To tune your RC model airplane engine, start with the engine off, and turn the HSNV clockwise till it stops. Turn it carefully and don’t force it as the valve is delicate. Then turn it counter clockwise 2 1/2 turns. Grab a hold of your RC model airplane and start the engine. Let it run for about 60 seconds, and then bring it to full throttle.
Quickly pinch the fuel tube attached to the carb. If the engine RPM increases, it means your engine is running rich, if it sags, or decreases, it means you are running lean. If the engine is rich, turn the HSNV clockwise 1/8 of a turn and pinch again.
Do this until the engine shows signs of being too lean, then back the needle off about an eight to a quarter of a turn and leave it there. If you find that the engine is already too lean from the start, turn the HSNV counter clockwise 1/8 at a time till the RPM increases when pinched, then continue counter-clockwise 1/8 again and leave it.
Low-speed needle valve
Only tune the LSNV after you are sure that your RC model airplane engine is broken in. Never try this on a brand new engine. Making sure that your RC model airplane is secured and will not go anywhere, start the engine and let it idle for 60 seconds.
Quickly advance the throttle from idle to full. If the RC model engine sputters and coughs but does not die it means that your LSNV is too rich. Turn the LSNV clockwise 1/8 and try again till the transition is smooth. If, on the other hand, your engine suddenly stops when you advance to full throttle, it means that your LSNV is set too lean. Turn the needle counter-clockwise 1/8 at a time till you find the sweet spot.
Get your RC model airplane in the air now. It should be tuned and ready to fly. Small adjustments may be necessary to your HSNV if you let the engine sit for extended periods, but a well-tuned engine should require no adjustment for a full day of flying once you have it set.
Here’s a good video from Youtube: