This is how I tune a gas engine. It takes time, but doesn't everything that you do the right way?

Lean the high end out untill it is maxed out. You can hear the engine pick up RPM and then stumble when it gets too lean. Set it right at max and then back it up the thickness of the slot in the adjustment screw. Maybe 1/16th turn on the screw. 1/8th turn is too much. You just don't want it set to peak RPM. But, you only need to richen it just a little bit. Gas needles are a lot more sensitive than glow needles.

Now set the low end like this:

What your going to do is rev it up to full throttle and then back it down to 1/2 throttle and listen. You'll back it down to 1/2 throttle and listen to it. If it's smooth for 10 seconds, then you can start to back it down 1 click at a time and listen to it. If it misses or gurgles--your too rich and will have to make a small adjustment. This usually involves stopping the motor. No big deal. Stop it. Lean the low end about the thickness of a screwdriver blade. Start it up and run it up to full again. Back it down to 1/2 and hold. Now back it down 1 click at a time. When does it gurgle and miss? 3 clicks down from 1/2 throttle? 6 clicks down? Just pay attention and take note of when it starts to act funny.

Each time you start it and run it up to full throttle you will hold it for about 10 seconds. Then back it down to 1/2 throttle and hold for 10 seconds. Back it down 1 click at a time. Hold it for 10 seconds each click. Listen to it and if it sounds clean--then back it down another click. Listen to it.

You should be able to back it down farther and farther each time you make an adjustment to the low speed needle.

Make an adjustment. Start it up. Full throttle. Back it down to 1/2 throttle and start counting clicks as you throttle down. Hold each click for 10 seconds. As you lean it and make adjustments--you should be able to back it down farther and farther each time before it gurgles and misses. When it gurgles and misses, take note of how many clicks you are down from 1/2 throttle and then make a small adjustment.

Eventually, you'll get it backed all the way down to an idle and it will be super smooth. Thats what you want. No mid range gurgle or missing.

When you get the low end set--you'll need to go back to the high end and check it. If you have to lean the low end a lot--usually that will effect the high end too and make it lean. So, if you made more than 1/2 turn lean on the low end to clean it up--you'll most likely have to go back and richen the high end again. Just get in the habit of going back and double-checking the high end after you set the low end.

Set your high side again. Set it to max and then back it up the thickness of the slot in the top of the screw. About 1/16th turn. Not more than 1/8th turn. It's not necessary to run it rich like a glow engine.

Thats the only way I know how to tune them. It works, but it's time consuming.

Some guys think that you'll fry the engine by running it up to full throttle on the ground and letting it run 1/2 throttle for extended periods of time. I disagree, as long as it's got air moving over it. If the cylinder is sticking out the cowl and you got prop blast on it--no problem. If it's inside a tight cowl and you don't have good airflow, then you'd be smart to shut it down as Silversurfer suggests and let it cool between adjustments. Most of my airplanes are so drastically overpowered that my engine cylinders are sticking well out of the cowl and they get plenty of airflow while I make my adjustments. I just start them up and let them idle for 1 or 2 minutes. Then I rev them up to full throttle for about 20 or 30 seconds to heat them up. After that I start making my adjustments to the carb. Get it dialed in and go fly it.

Make all adjustments with the cowl on. If you get it all tuned up without a cowl--you'll just have to do it again after you put the cowl back on. The cowl effects the way the air flows into and around the carb. Leave it on.

An engine that surges up and down at high speed is lean. Richen the high speed needle 1/4 turn and try it again.