The black screw on the FRONT of the carb is an AIR-BLEED screw. It's your idle mixture screw. Not to be mistaken for an idle SPEED screw. This Air-Bleed screw meters the incoming air at idle.

Just so we're clear--you need to know the difference between "RICH" and "LEAN"

A rich mixture--be it on the top end or at idle--means that there is too much fuel in the engine, and not enough air.

A lean mixture means exactly opposite. Too much air and not enough fuel. This is dangerous. Never run an engine too lean. You'll cook it pretty quickly.

The engine should be fully broken in--say 3 or 4 tanks.

Start it and run it up to MAX RPM. Turn the high speed needle in--clockwise--and listen to the motor rev up as you lean it out. By turning the needle in--you are shutting off the fuel flow a bit and allowing the engine to suck more air into each fuel charge.

As you turn the high speed needle in--the engine will gradually pick up speed and really start coming alive. Continue to turn the high speed needle in a click or 2 at one time and listen to the engine pick up speed.

At some point--you will turn the needle a few clicks and nothing will happen. You've just reached MAX LEAN. If you continue to turn the needle in--the engine will sag and loose RPMs. It may even die. If it does--just back the needle out about 1/4 turn or 1/2 turn--restart the engine--and start over.

So--when you reach MAX LEAN--you should then turn the needle OUT a few clicks untill the engine slows down again. You don't want to run it at MAX LEAN. You need to turn the needle back out--richen the mixture--untill you can hear a noticable drop in RPM. A tach is good here. You want to be about 500-800RPM off MAX LEAN. You want it to be slightly rich or you'll cook it.

Now for the low end. We are going to adjust the air-bleed screw. The air-bleed screw works by opening and closing a little hole in the front of the carb. If you screw it in--then it closes the hole--thereby reducing incoming air, and making the idle mixture RICH. If you screw it out--you are increasing the incoming air at idle and making the mixture LEAN.

You want the engine to be able to idle for extended periods of time. It should idle for AT LEAST 30 seconds without problems and then if you were to push the throttle stick rapidly forward--the engine should instantly pick up and run full throttle. It's called throttle response--or transition.

So, let it idle for 10-15 seconds and see how it reacts. Push the throttle forward rapidly and see how it does. Does it throttle up instantly without any hesitation?

If it dies when you push the throttle stick forward--you got problems.

Here's how to diagnose the issue:

Don't touch the high speed needle again. It's set--forget about it.

If it idles for 15 seconds and coughs and belches raw fuel out the exhaust when you throttle up--then it's RICH. You need to turn the air-bleed screw OUT to let in more air at idle. By not letting in enough air--the engine is drawing in more fuel than it can burn. So--when you throttle up, the excess fuel enters the combustion chamber and causes rough running for a few seconds while the engine "catches up"

If it idles for 15 seconds and dies almost instantly when you throttle up--then it's LEAN. You need to screw the air-bleed screw IN to choke off some of the incoming air and get more fuel into the mixture. A lean engine will run for awhile at idle. You can tell it's lean if you just sit back and watch it idle. A lean engine will idle fine--but it will VERY SLOWLY gain RPMs. Then suddenly and abruptly, it will just die. Thats because it ran out of fuel. Plenty of air--actually too much--and not enough fuel.

A rich engine will idle fine for a few seconds. Maybe 10-20 seconds. But it will slowly start to run rough and shake a bit. Its getting flooded. It can't burn all the fuel. It will slowly--about 10-15 seconds--loose RPM and finally gurgle up and die on you.

Be carefull using an electric starter. Liquids don't compress--like a vapor. If you get it rich, and it dies--then there's excess fuel inside the engine. It won't compress--so when you hit the engine with a starter--the fuel gets pushed up into the combustion chamber--on top of the piston. When the piston comes up and can't compress the liquid fuel--it can get severely damaged internally.

To avoid this--simply pick the plane up and turn the exhaust port--muffler side--down towards the ground and wiggle or bounce the prop back and forth a couple times. DON"T pull it through compression. Wiggleing the prop back and forth--and having the muffler pointed down--will get the excess fuel up and out the muffler. Let it drip/pour out.

Then try and prop it over by hand. Obviously--the ignighter is NOT connected. If you can turn it over with your hand--after draining it--then you should be okay with the starter.

The airbleed screw is only effective/working when the carb is in an idleing position. When your at full throttle, the air-bleed hole is closed off and your sucking all the air through the top of the carb.

Good luck. Hope this helps.