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Like any other mechanical and social activity, model flying has its own technical and behavioral “secrets” that more experienced model pilots take for granted. In fact, most of these “Old Pros” take so many things as just the “way it should be done” that they forget that newer pilots really don’t know these great truths.
Explaining some of these “great truths” is what we are going to attempt here. In this and in future articles, we’ll cover tuning your engine, battery management, starting techniques, landing gear setup, fuel selection, transmitter dual-rate setup, radio range checking, throttle setup, field rules, field etiquette and dozens of other topics. For now, it might be best to start at the front of the airplane and work towards the rear. That means its time for:
Engines
Today’s model 2-stroke engines, especially those in the .25 to .61 cu. in. displacement range are nearly works of art. While incredibly difficult to design, these engines are simplicity itself to operate. If the glow plug is working and there is the proper ratio of fuel/air mixture in the combustion chamber, the engine WILL start. There are no other requirements. Well, someone does have to rotate the propeller, but that is the only extra thing that is required. If the high and low-speed mixture controls are properly set and the fuel is good, the engine will always run until the tank is empty.
So why then, is it so difficult to get the engine running reliably? Maybe it is poorly set mixture controls. A two-stroke model engine is fuel/air mixture dependent. That is, nearly its entire operating system depends on having the right fuel to air ratio. Properly setting these mixtures requires just a little understanding of how the mixture controls are designed to work in sport engines.
Mixture Settings
The high-speed mixture control, called a needle valve since the section inside the engine tapers to a sharp point (photos 1 and 2), meters the amount of fuel that flows into the carburetor at high speed. For most practical purposes, the mixture setting determined by this control has little effect on the engine’s actual mixture until the throttle is opened more than 40% or so. Therefore, do not use this control to set the engine’s mixture below half throttle.
 
Photo 1 Photo 2
Instead use the idle, or low-speed mixture control. There are two types of idle control as shown in photos 3 and 4. The most common is the separate screw or small needle. But the second, “air bleed” type is also common. Both work effectively, but only at throttle settings below 60%. Therefore, do not use this control to set the high-speed mixtures.
 
Photo 3 Photo 4
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