Model Engine Company of America
2-3-2020
How to adjust needle valves on aircraft engines
The number of turns out on a main needle is an arbitrary starting point. No set number of turns will be correct as the setting will vary with fuel, air density, elevation, humidity and temperature. You have to learn to tune your engine.
A good starting point is 3 to 5 turns out on the high speed and and the low speed should be backed out to a point where it is not affecting the high speed mixture.
A model engine makes sounds that will tell you how it's performing. You'll have to listen very carefully for them, recognize their message, and make adjustments to the fuel control needle valves accordingly. The mixture of fuel and air is controlled by the amount of fuel metered by the needle valve.

SLOPPY RICH MIXTURE running is characterized by a very slow, irregular, sputtering exhaust sound. The exhaust gas will be very smoky and contain many droplets of oil. NEVER RUN AN ABC ENGINE AT THIS SETTING. The cylinder is not able to heat properly and the fit will be ruined in a short time.

RICH MIXTURE running is characterized by a slower, sometimes irregular, sputtering exhaust sound. The exhaust gas will be smoky and probably contain small droplets of oil. NEVER RUN AN ABC ENGINE AT THIS SETTING. The cylinder is not able to heat properly and the fit will be ruined in a short time.

FOUR CYCLING or SLIGHTLY RICH running is a rich type setting, but it is fast enough to pull the airplane. This is the setting you normally look for before launching the airplane because the engine will run leaner when airborne.

PEAKED OR TWO CYCLE. As the main needle is closed (clockwise), it reduces the amount of fuel mixed with the air drawn into the engine. At a specific point, which varies with each engine, air temperature, altitude and relative humidity, the exhaust note will change quickly into a smooth, powerful note. If the needle is closed further, the note will stay smooth, but will weaken. The peak occurs just at the break point from a rich setting and further leaning will ruin the engine. A lean setting raises the engine heat above the safe point, reduces lubrication, and destroys glow plugs due to high combustion temperature. This is very harmful to the engine and your investment. Learn to tune the engine before flying. Remember, a little rich is always preferred for long motor life.

 

Here are the frequently asked questions...

How many turns outs should my needle be set. Turns out are an arbitrary starting point and  should not be considered a correct setting. If we knew exactly how many turns out a needle should be set you wouldn't need a needle to adjust. You have to follow procedures below to obtain the correct settings.
How do I set the high speed adjustment?

First be sure the idle needle is not interfering with the high speed adjustment as described in the next step.

If you have a new engine read How to break in an ABC engine or How to break in a Ringed engine

After the engine starts open the throttle to the full open position.
Unscrew the main needle...
if the engine slows, it is on the rich side of the setting, slowly turn the needle in until the desired mixture is achieved as described above.

if it speeds up, it is on the lean side of the setting, continue unscrewing the needle until the engine begins to slow. Now you are on the rich side of the setting, slowly turn the needle in until the desired mixture is achieved as described above.

Can't get low speed adjustment right.

For carbs with a low speed needle in the center of the throttle barrel like on all the MECOA's & GMS's:

The high speed must be adjusted correctly before the idle mixture can be set.

Unscrew the idle needle until it stops or the head of the needle is flush with out side of the receptacle it screws into. At this point we know the idle mixture will be very rich and not interfering with the high speed needle. Start the engine and open the throttle to full speed. Adjust the high speed needle to the correct setting. Close the throttle to the lowest possible speed without the engine stopping. Now screw the idle needle in, the engine should speed up. Again close the throttle to the lowest possible speed without the engine stopping. Screw the idle needle in again, the engine should speed up. Keep repeating this procedure until a low idle is archived.

Using this method, you will always obtain the correct idle setting because you will be starting from rich setting moving towards a leaner correct setting.

Engine leans out after running for a while. This is usually caused by a pin hole in the pick up tube inside the fuel tank. Check for air bubbles in the fuel line with the tank nearing empty while the engine is running. If you see bubbles when the fuel level is low and not when the tank is full there is a pin hole that is being exposed when the fuel drops below it exposing the hole to air. Replace the fuel line. 
Engine leans out, then richens. Mixture never stays correct.

This is typically caused by debris in the carb plugging and unplugging the needle orifice. This causes irregular mixture setting at high RPM's. Disassemble the carb and clean in the main needle passage and spray bar.

Another cause of this is the pressure tap in the muffler. Some oils in fuels can cause the tap to be plugged up not allowing pressure to the tank and in turn depriving the engine of fuel, then the oil will blow through the line providing pressure to the tank providing more fuel to the engine. Hence, pressure, no pressure, as you fight to find the correct adjustment.
Engine leans out when it gets hot. This is sometimes caused by loose head screws. Tighten all the head screws in a cross torquing pattern.
Fine tuning the idle mixture. Start the engine and open the carburetor to the full open position, then adjust for peak R.P.M. with the main needle as previously described.

Close the carburetor barrel slowly until the lowest possible speed is reached without the engine stopping.

Go to full throttle after about 10 seconds of idling. If the engine gains speed slowly, the idle mixture is too rich. If the engine stops, the idle mixture is too lean. Turn the idle needle clockwise if mixture is too rich and counterclockwise if too lean.

The engine will accelerate from idle to full throttle smoothly and instantaneously when properly adjusted. The engine may not idle well at a low setting or accelerate as quickly until it is has some running time on it.

 
 
The above information is provided as a guide. Since MECOA/K&B has no way of determining the ability of the individual using and understanding this information, we assume absolutely NO RESPONSIBILITY for any damage to person or property from the use of this information.