Before flying, the idle mixture needs adjusting. Most .40-size engines use a separate idle needle valve. The idle adjustment screw or needle valve meters the amount of fuel that flows into the carburetor during idle. Before adjusting the idle mixture, make sure this valve is set per the engine's instructions. Clockwise adjustments lean the idle mixture and counterclockwise turns richen it.

Some engines use an air-bleed hole located in the carburetor's top front section. A screw meters the amount of air admitted through this hole at idle, adjusting the idle mixture. Initially the screw should cover just half of the air-inlet hole (see photo). This may be too rich, but you can lean the idle mixture by turning the screw clockwise. Turning the screw past the hole continues to adjust the idle mixture, despite appearances.

There is little purpose in adjusting the idle mixture on the test stand since fuel pressure, air-intake volume, and airflow will be different once the engine is installed in the airplane. The idle setting will have to be readjusted again.

Mount the engine in the airplane if you have not already done so. Run the engine at full throttle, and set the needle valve slightly leaner than the alternating sound point. Stop the engine, attach the glow driver, and restart it.

Slow the engine to approximately 3,000 rpm (a tachometer helps here). Watch the rpm. If the engine gradually slows and then stops, the mixture is too rich. Once the engine stops, lean the idle mixture one-quarter turn and restart. If the engine rpm increases, the mixture is too lean. Richen the idle mixture, again once the engine is not running, one-quarter turn.

Check each new setting by running the engine at full throttle and then reducing to 3,000 rpm. This "clears" the previous incorrect idle setting. Even if the engine does not quit but needs final adjustment, stop it before making idle changes. Take every opportunity to stay away from a spinning propeller with your hands or screwdriver.

Continue adjusting until the engine holds a steady 3,000 rpm. Disconnect the glow driver and make any final idle adjustments. Why have the glow driver connected during the initial idle settings? Incorrect idle mixtures often dampen an unconnected glow plug so quickly that there is no time to determine what is wrong with the setting. Keeping the plug "lit" helps ease the adjustment process.

After the initial settings, disconnect the glow driver, idle the engine for 30 seconds, and then quickly advance the throttle. If the engine stops, richen the idle mixture slightly. If the engine stumbles and quits, won't accelerate, or accelerates exceptionally slowly, lean it a bit.

During the first few flights, 3,000 rpm provides a reliable idle for most engines. Slower idle settings are possible but run the risk of the engine's quitting because of the high internal friction during break-in. Set the initial throttle trim on the transmitter for a 3,000 rpm idle at full "up" throttle trim, and full "down" throttle trim stops the engine.

Landing patterns are flown at high idle. Once the field is "made" (the model can glide to the runway without engine power), reduce the trim to half. If the engine quits, landing is no problem. If it runs more slowly, you'll make a pretty landing. This half-trim setting will be roughly 2,200-2,400 rpm and is the target idle speed once the engine is fully broken in.

Breaking in an ABC engine is somewhat easier. Only one ground run of 10-15 minutes is required, using the flying propeller. Set the high-speed needle valve to the most open setting cited in the instructions. Start the engine at full throttle.

The exhaust sound should be slightly lean of the alternating low- and high-pitched sounds. If you hear only a high-pitched sound, richen the mixture. If you hear only a low-pitched sound, lean the mixture to just past the alternating point. Run the engine for five minutes, alternating between full and half throttle.

Run the engine for another five minutes at a slightly leaner mixture setting, again alternating between full and mid-throttle. During the final five minutes, lean the high-speed mixture until rpm peak and start to drop. Immediately richen the mixture to 1,000 rpm less than that peak (roughly a half turn). This is the initial flying high-speed mixture. Adjust the idle mixture just as for ringed engines.

After approximately 10 flights for ABC engines and 20 flights for ringed engines, the high-speed mixture can be leaned to 500 rpm less than peak. Never run leaner than this. A trainer's engine turns approximately 500 rpm faster in flight than on the ground. The mixture tends to lean as rpm increases. In steep climbs and while inverted, fuel feed rates are reduced. Most important, fuel pressure drops as the tank empties, even with muffler pressure, as the weight of the fuel pushing itself into the fuel outlet (tank head pressure) gets lower.

The slightly rich ground mixture compensates for all these possible problems. A setting of 500 rpm rich is the leanest run without a fuel pump, but 600 is better and will greatly lengthen engine life.

Next month "From the Ground Up" will look at fine-tuning propeller, fuel, glow-plug, and engine-size choices. I'll also show you some of the differences with four-strokes. MA