Engine tuning 101, or maybe a source for some troubleshooting ideas?

Some thoughts prior to starting the first time (or starting over again?):
The idle adjusting screw is the one closest to the engine. The high speed furthest away. Turning them in (clockwise) will lean them, out (counter clockwise) will richen.

If you have the factory suggested starting settings, set them there now. If you don't, lets set them both at 1.5 turns from closed. Be gentle when closing these screws completely as they are pretty small and not terrible difficult to damage. This can/will cause adjustments to be erratic from that point on.

Please don't try the first time with the cowl installed. You need to see the fuel line, and easy access to the adjusting scews make things much simpler.

Idle speed screw should have been removed when you hooked up the servo. That's been done, right? You'll be using the radio to adjust your idle speed.

Prop is set so it's at about 1:30 when it comes up on the compression stroke, right? Much easier to flip from here while keeping your balance and your hand out of the prop.

If you've never run the engine, or it's been a long time - a dry carb has a terrible time pulling fuel. The check valves need to be wet with fuel to function properly. Because of that you may want to be prepared to prime the engine through the carb. Another way is to pull the side of the carb held in place by the single screw, and carefully flood the carb with mixed fuel the best you can.

To help the engine draw fuel more easily when the choke is on, closing the small hole in the choke plate has proven very effective. Solder works well, but use what you want. If not solder, consider what might happen to whatever your using if it were to become dislodged and be sucked into the engine. Sealants that remains soft won't cause any damage. Use no more than absolutely necessary to close the hole. The hole is not required in this application.

Starting:

So, choke closed, low and high screws set per above, start flipping while watching for the fuel being drawn up through the fuel line. If you haven't seen the fuel and nothing has happened after 15-20 flips, the carb is dry! See above about flooding the carb with fuel to wet the check valves, or get your primer out and use that to shoot a squirt of fuel into the carb's opening.

Once you hear the engine start for a second and quit, flip the choke off and it should start again within just a few flips. If it starts and quits again, and you still haven't seen fuel in the fuel line, prime again! You shouldn't have to do this more than a couple of times, and then, only if the carb is dry.

Engine is running:

Once started, let it warm a bit, gradually increasing rpms as it does. After a minute or 2 take it up to full rpm. and hold it there. Adjust the high speed for maximum rpm, then richen until you hear it slow just a bit (maybe 200 rpm if you're using a tach).

Slow the engine down to something near 1/4 throttle and let it cool for a second, then lower to idle speed. Idle speed is when your plane is right on the edge of trying to move on it's own, not much less. Something in the mid to high teens if you're using a tach. Adjust for max idle rpm, if it starts exceeding idle speed, adjust idle speed as necessary to keep it in line. After getting that set, check your transition. Quickly run it back up to wide open to see what happens. If it quits, richen the low speed a little (a little = a screwdriver blade width), and restart. Let it warm for a second, then lower to idle and take it to wide open again. If it quits, repeat the process until you get snappy response (transition) when opening it to wide open.

Once proper throttle response is achieved, go back to wide open and reset that, using the same process as above, leaving it slightly rich again. Try your idle. If that's ok and your throttle response is still good, it's time to go flying. Do that. Further adjustments will be required no matter how carefull you are here on the ground. More is just a waste of time. That's a given. If you're building confidence just messing with it, that's a different story. Knock yourself out, but it's not necessary.

In the air, finally!

There's so many variables at this point, this is just a collection of notes I use frequently. All assume the tuning process above has been done - recently! The engine also has a reasonably fresh and properly gapped plug installed, and the crap trap in the carb is confirmed clean!

A rich engine won't quit!
First flights with carb settings set slightly rich (without being slobbery rich!) are generally safe ones, big confidence builders. Setting up this way intentionally for the first few flights is cheap insurance.

If your engine is quitting, stopping completely, and you are absolutely sure it's not sucking air through the fuel line, it's lean. It's that simple. If it happens when you pull back on the stick, it's probably your low speed adjustment. At wide open (no matter the attitude) it's the high speed. There's also the potential other stuff can be in play, but assuming it's lean to start generally pays off 99% of the time. When dealing with a lean condition, keep in mind that fuel lines sucking air are common. Really be carefull with this one before getting too carried away. A LOT of problems are traced back to fuel lines sucking air. Guys replace carbs prior to realizing this on occasion....

High idle hang?
Your engine is running great, right on the edge of lean, making max power at wide open throttle. Time to bring it in, you pull back on the stick, but the engine does not come all the way back down to idle for several seconds, maybe up to 30 seconds? Here's what's happening. The combustion chamber is hot from being run. Not hot to the point of being hurt, but on the edge (that's where it needs to be to make max power). By pulling the stick back to idle, you've robbed it of a major cooling source - it's fuel, and the air coming in with it. It's going to need a few seconds to cool down when the only cooling it's getting is the air going through it's fins. That's your delay (the high idle hang)! You can hurry this cool down process by throwing some fuel in there, and that can be done with your idle mixture. Going richer with that can hasten that cooling process, possibly to the point of eliminating it all together. Richening the low speed to that extent will very likely require you to raise the idle speed trim, and the resulting idle will be slightly lumpy. Not totally unexpected on a 2 stroke engine, and nothing that's going to cause it to load up and quit. Adjust a little at a time until the hang problem is lowered to acceptable limits, or eliminated altogether.

There's another option here as well. Remember what's causing this to start with, running on the edge of lean on your high speed? Going rich enough on your high speed so the combustion chamber won't get as hot is another solution. Myself, I'll take the lumpy idle solution...

Mid range burble?
There's nothing wrong running it like this, but if it's making you crazy -
I would start by opening the spark plug gap to .024. Sounds nuts I know, but try it. That's been proven to make a noticeable difference. Next is leaning the high speed mixture, a fraction at a time! If the engine starts quitting on you, obviously you've gone too far. Another indication you've gone to far might be you've suddenly picked up a dose of that idle hang (above). That may not be a bad thing, just something else to deal with.

There's another line of thought on the burble as well. That is the burbling is occurring because the engine is so lightly loaded. 3D planes running low pitch props on planes that don't weigh anything are a good example of burble that can be really hard to get rid of. A sport plane might be able to go to the next size bigger prop and make a difference?

Poor low end response?
Moving the stick up from idle doesn't cause a big bog or the engine to quit, but the engine just takes it's time to accelerate. Top end, from 1/2-3/4 throttle on up seems fine? Might be a little rough sounding on this initial acceleration, but maybe not. It's probably rich on the low speed adjustment. Good first place to look anyway.

Hope this helps!

-Al