Products - Frequently Asked Questions (FAQ)
Practical Needle Setting
Any model engine, once it can be run on the test stand while leaned out to peak RPM, can be used in any application. When used in an airplane, the needle is leaned until the engine is at peak, and then richened somewhat. This extra bit of richening is done because the engine will have a tendency to lean a bit as the fuel tank is drained.
The engine leans as the fuel tank is drained because a full tank has a higher fuel pressure than an empty tank. The difference in the height of the fuel between a full and an empty tank causes this. It's called a change in the fuel's "head" pressure. As the tank drains, the head pressure will decrease. High-"G" maneuvers will also cause fuel pressure to lower. The high "G's" will pull the fuel away from the engine, which leans the mixture.
Running an engine just rich enough to prevent it from going past peak is the goal. That way, it will never run too lean in the air.
As different weather patterns move across an area, atmospheric pressure will change. Therefore you cannot rely upon an engine to keep the same needle setting from day-to-day. It must be checked every time the engine is run. Sometimes, the setting can change dramatically during the day, as air temperature and humidity change. It's a never-ending battle to keep the engine properly-set and healthy.
You can always tell if an engine is leaned to the rich side of peak RPM by using the "pinch" method. Just give the fuel line a pinch and quickly let go. If the engine's RPM increases, you can lean the engine further. If the RPM remains the same or drops, you must immediately richen the engine...it's too lean. Generally, you want to have a slight increase when you pinch the line and the engine's running at full throttle.
If you cannot get to the fuel line because the engine's cowled, the best way to check for proper leaning is to run the engine at full throttle with the fuel tank about 1/4 full. Lean the engine to the desired point and then pick up the model and hold the nose straight up. The engine should pick up in RPM. If it picks up a lot, you can lean the engine a bit more. If it stays the same or drops, you must immediately richen the engine.
Because the engine tends to lean as the fuel level in the tank drops, adjusting the engine for conditions near the end of the flight will keep you from damaging your engine.
If you then fill the tank and your engine runs very rich at the setting you just found, then you have a fuel tank location or fuel feed problem. Large mixture changes during a flight are not good. You'll need to find out what is causing the problem.
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