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Modern two-stroke nitro engines have become more and more reliable over the years, but they still take some time, patience, and practice to tune correctly. Let's see the basic fundamentals of tuning your Heli's engine for high performance and reliability every time. And remember, practice makes perfect!


Before You Begin....
Proper engine break-in is the key to a long lasting engine. Breaking in an engine is an article unto itself, so for the purposes of this article we're going to assume that your engine has already been broken in correctly. Also, always observe common sense safety rules and AMA guidelines when making any adjustments to your helicopter.

Identifying Lean, Rich, and Just Right
There are three engine conditions that you should become familiar with before you attempt to tune your engine. Here's a quick description of each.


Lean describes a situation in which the ratio of fuel to air entering the carburetor is heavily favoring air. This means that there's not enough fuel mixing with the air as it enters the engine. This condition will cause the engine to heat up prematurely and run too hot because of a lack of lubrication. You can identify a lean engine by the overall lack of smoke coming from the exhaust, as well as a crisp sound to the engine accompanied with a popping sound, and you may notice that when you reduce the throttle the RPMs stay up based on the sound of the engine.


Not surprisingly, rich is the opposite of lean. When an engine is running rich there's too much fuel going into the carb as compared to the air flow. This will not hurt an engine like running lean can, but it can stall the engine and lead to an overall lack of performance. You can identify a rich condition by an excessive amount of exhaust smoke and/or oil/fuel, a lack of responsiveness on throttle up, and an overall sound as though the engine never seems to clear out.

Just Right

Somewhere between rich and lean lies "just right." This is the spot that you are always looking for when you tune. Just right can be identified by a moderate amount of smoke, the engine having a nice constant sound as you climb, and the engine speed sounding as though it's in perfect tune with your throttle input.


Where to Begin
Your engine should have come with an instruction manual that has some basic high end and low end adjustment points for the carburetor. This is always a good place to begin. Other variables like fuel, glow plug, and weather conditions will affect this starting point, but it's usually a safe place to start.

Getting Familiar with the Carb
Most carburetors have high-end and low-end needle adjustment. The high-end adjustment determines the overall amount of fuel flowing into the engine at every point along its power band. The low-end needle adjusts the flow of fuel during idle and low RPM.  Although less common, some engines have a third needle which is the mid range. For the purposes of this article we'll concentrate on the low and high speed needles.   






The Pinch Test
Once you start the engine, allow it to run for a few minutes at idle so that the real operating temperature is reached. Trying to adjust an engine that's below operating temperature is a waste of time, since the performance will change with the temperature. After a minute or so of idle clear out the engine by applying some throttle then back at idle give the fuel line leading into the engine a pinch to cut off the flow. If the engine dies within a couple of seconds it's too lean, so richen up the low end a couple of clicks. If the engine continues to run for more than a few seconds it's too rich, so lean it out by a couple of clicks. The engine should run for 2 seconds, speed up, then begin to die. Repeat the pinch test until you've reached this desired result.

In Flight Testing


Once you see that the engine is not stalling at idle and you're comfortable with how the carb is set, bring her up to speed and get the Heli into a nice hover. Look and listen, and note whether or not the engine appears to be either too rich or too lean. If it's lean, richen the high-end needle 2 clicks. If it appears to be too rich, lean the high-end needle two clicks. Take your time, and don't go more than two clicks at a time. Once you've found what you feel to be just right, move onto climb.


When it's idling well, it looks good when hovering, and now it's time to climb. At a hover, stab the throttle to near full power and see how the engine appears during your ascent. If there was hesitation or a load of smoke, then you're probably still a little rich and will need to lean out the high speed by a couple of clicks. If the Heli shot upward instantly, had little or no smoke trail, or sounded as if it were popping, then it's too lean and you'll need to richen it up by a click or two. As with the steps before, repeat the process until you find the setting that's just right.

Pinch It Again

Because the high-end needle affects the entire range of the engine, it's a good idea to go back and repeat the pinch test after you're comfortable with hovering and climbing.


The process illustrated above is the same no matter what the conditions, although there are some things you should know about the ambient air temperature and humidity. Every time your engine goes through a cycle it draws in a fixed volume of air. Within that volume of air are oxygen molecules, and oxygen is necessary for your engine to run. When the air is humid, there is less oxygen within that same volume of air, meaning that you'll tend to run the engine more lean than rich. When humidity is lower there's more oxygen in that same volume of air, meaning you'll want to run richer so that you achieve the optimum ratio of fuel to air (oxygen). Temperature also plays a role, but it's less significant than humidity. As ambient air temperature increases the air becomes less dense, meaning less oxygen. You would tune just as you would for more humid conditions.

Engine tuning is not an exact science, but with some practice and patience you'll get comfortable with your skill and be able tune for any conditions mother nature throws at you. Throughout your flying day if you see the conditions change, or your engine seems to have fallen out of tune, adjust accordingly and have fun.

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