By Jerry Olenik of Green Sky Adventures As promised, I am going to go through some things that may help with properly tuning the 2-stroke aircraft engine.
No matter who makes the engine, the first thing I do is lean the idle mixture. Even if you think you have a smooth running engine at idle, you may still get problems at other power settings from excessive fuel that loads up or even harmful long term effects like excessive carbon build up. Often people go changing main jets or needle clip positions because of fowling spark plugs or whatever when the problem is really coming from a rich idle.
Most of the time with a 2-stroke, you can not lean to peak efficiency like in a Cessna or the like because the engine will seize before you get there. However, at low engine speed you CAN lean to peak efficiency without damaging the engine. To do this, I would suggest securing the airplane so it can't roll away, because as you lean the mixture, your idle speed is going to increase and may increase enough to make the plane roll away.
Once appropriate measures are taken to secure the plane, start the engine and listen carefully at idle. Listed for occasional bumps or misses. Listen to the general tone of the engine. Feel the airframe for vibration. This is sort of like tuning a guitar by ear. Then shut it down. The 2SI engine really is usually a smooth running engine at idle even when it's too rich, so you may have to go by RPM more than the smoothness. RPM will increase as you get closer to peak efficiency. With the Bing 84 carb or Bing 54 carb, start turning the idle air screw out (counter-clockwise) about 1/2 turn (180 degrees). This would be the smaller screw that is recessed into the carb. Turning it out allows in more air and effectively leans the mixture in the idle RPM range. As you turn it out, keep track of how far you have gone. After each half turn, start up the engine and listen and feel for bumps, misses, roughness and the general tone of the engine. Generally a deeper tone will be richer and a higher pitched tone will be leaner. Continue to go through this until the engine seems to be running completely smooth. As smooth as glass. Note where you are on the air screw adjustment at that point.
One thing you may run into is that you could run out of adjustment on the airscrew. If you turn it out far enough to see the o-ring that seals it, you need to go to the next leaner idler jet. If you change to a leaner idler jet, start out with the airscrew turned out about 1/2 turn and continue with the process. Even if it seems like it is smoothed out, note that setting and continue. You will eventually get to a point where the engine will decrease in RPM and/or die or not want to run. Note that point as well. I would then set your idle air screw about mid way between where it seemed to totally smooth out and where it was too lean to run. That way you should not need more adjustments with changes in the weather or density altitude unless those changes are severe.
Now that you have your idle set, you are ready to try your mid range. NOTE: leaning the idle can sometimes have some effect on the midrange and you may want to start out in the richest (lowest clip position) needle setting until you know where your EGT is going to be. With the engine properly warmed up, increase the throttle into the 4000 - 5000 RPM range. Make movements with the throttle watching your EGT for the hottest spot and just make a mental note of where your engine has it's hottest EGT. Be cautious about staying within limits on all your temperatures because now you are running at engine speeds that can cause damage if mistakes are made.
For reliability and engine longevity reasons, I take exactly the opposite approach to mixture at higher power settings as I do for idle mixture settings. What I mean is that I like to make the mixture rich until it runs a little bit rough, then lean it out a little from there. This is just the opposite that you would do a in Cessna or other GA type aircraft. While you are searching for that hottest spot in your mid range, keep your ears open for any little misses or bumps, and feel for any momentary waves of vibration in the airframe. Those are all signs that the mixture is bordering on being too rich. The reason you are looking for the hottest spot in your EGT is so that you don't make an adjustment that would put that over limits. Each position on the jet needle will change the mid range EGT approximately 50 degrees + or -. If you don't notice any roughness or other signs of being too rich in the mid range. Make it a little richer. If you are already in the richest position on the jet need and well within temp limits, you might just leave it alone here. Otherwise you would need to go to a different needle jet to make the mixture richer. If the mid range is rough and acts like it is rich and you are well within limits on your EGT, you can move the clip to the next leanest position. When leaning the mixture always make small incremental changes because too lean will cause damage often without warning. So if your EGT is 900F or 1000F and the engine is running rough in the mid range, try moving to the next leaner needle position. Do this until either the engine smoothes out in that range or you come within 50F of your upper EGT limit. If your EGT limit is 1200F and you are at 1150F, you probably do not want to make another adjustment any leaner.
Once the mid range is set, with the engine properly warmed up, you can do a static run up to full power. Make a note of the maximum RPM and the EGT. If your engine makes its rated power at 6000 RPM, you maximum static RPM should usually be a little less than that. How much less really depends on how fast the aircraft is because that max RPM will increase during flight. So if you have a relatively slow aircraft with an engine that makes it's rated power at 6000 RPM, you might expect to set your maximum static RPM to 5800 or so. If you have an engine that makes it's rated power at 6500 RPM, you might set your maximum static RPM at 6300 or so. So if you are not close to that, you should make a propeller pitch adjustment to load the engine properly. If you do have to make a large change in propeller pitch, it will also have an effect on your fuel-air mixture throughout the entire RPM range of the engine. So if you were loaded to 6300 and had to make it 5800, your mixture is going to get richer. If you were loaded to 5800 and you need to change it to 6300, your mixture is going to get leaner. So after adjusting your pitch, it is a good idea to go through the carb adjustment process again.
If your full power EGT is fairly low, like in the 800 range, that may also be having an effect on your full power output. Generally, you want the full power range to have a lower EGT than the mid range for added protection under the higher stress conditions, but if it is too much lower, the engine will not perform. If your EGT seems to be very low at full power, and the engine seems to bog down, you may need to change to a leaner main jet. The main jet is the only part of the carburetor that has control over the full power mixture. At full power, the jet needle is all the way out of the jet and a needle adjustment will not change this setting. The main jet, however, WILL have some effect on the mid range mixture. So if you change to a leaner main jet, it is always a good idea to change the needle clip to the richest position on the needle for the first run up. You can always make the mixture leaner in the mid range if its too rich, but if it's too lean, you might need a new piston.
Since mixture effects the power output of the engine and propeller loading effects mixture, this process is sort of like hitting a moving target in some cases. If you find that you have to make a large propeller adjustment, you may have to go back and forth through this a couple of times. Once set, and set properly, however, you should be able to enjoy many hours of enjoyable, trouble free use. If you don't set the engine, carbs, and propeller up properly, you will likely have many hours of problems, frustration, and may even cause damage to the engine and/or airframe.
These are the exact same steps I use when setting up an engine on a customer's aircraft weather the engine is new or used. My customers are usually very pleased with the results since they no longer have to put up with the vibrations and throttle response problems that they just took as normal before. Hopefully you guys can get the same results.