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. A 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 set correctly, the mixture will have just enough fuel and air to allow the engine to run at peak performance. If incorrectly set, with either too much or too little fuel so that it is said to run either 'rich' or 'lean' the engine might sputter and/or overheat and die. You are ten left with recovering and landing your plane "deadstick" without power and run the chance of crashing. So you can see how important it is to tune your engine properly.
High speed needle valve
The first valve that is usually located on the outside of the carb with a long adjustable handle is called the High Speed Needle Valve (HSNV). This needle valve regulates the fuel mixture when the throttle is completely open and the engine is running at peak RPM. The second valve, usually located inside the throttle arm assembly, is called the Low Speed Needle Valve (LSNV). This needle valve regulates the fuel/air mixture that is present when your engine is idling or progressing from a low speed to a high speed.
To tune your RC model airplane engine, start with the engine off, and turn the HSNV clockwise till it stops, then turn it counter clockwise 2.5 turns. Grab a hold of your RC model airplane and start the engine. Let it run for about 60 seconds, 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 1/8 to 1/4 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.
