Ok, some helpful techniques that I use:

- Set the high speed needle first. On most engines, the low speed setting will be affected by the high end, and vice versa, but the effects of the low end on the high are less pronounced. (yes, the high and low speed mixtures DO affect each other to some extent. But it does vary a lot by carberator design. The non-airbleed OS carb designs are pretty good about the low speed not causing the high end mixture to change and vice versa. Some other carbs have more cross-effets (say what you want about OS engines, but their carbs are amoung some of the easiest to make work right, and many funky-running engines can be "fixed" by mounting an OS carb to it)).

- When I set the high speed, I run it up to full power, let it stabalize for a moment, then I pinch the fuel line with a solid pinch-and-release. This causes the fuel to drop off for a split second. Listen to the engine, it should pick up RPM and then settle back to the origional RPM. If no RPM change happened, either you didn't pinch enough, or the mixture is right on the edge of being too lean. Pinch again to verify, a little longer. If the RPM drops at all, you are too lean (or you held the pinch way way too long, but even then, if you clamp off the line, the RPM should pick up, the sag and die. If it didn't pick up to start with, you were too lean). How much pickup you should hear depends entirely on the engine/prop/etc, but you should hear some pickup. Unless you are marginal on power, or in a competition, backing off a little on the rich side is good for the engine anyway, so make sure you hear a good RPM change.

- Then set the low speed. You can do the same pinch test with the engine at idle. But I usually don't do it that way. Instead, I idle the engine down, hold a second, then pop the stick to full throttle. The engine should respond very quickly. If it lags and sputters, it's too rich. If it suddenly dies, it's probably too lean. But if it's really rich, it will often just die as well. So try again, bringing the engine up slower, and/or holding the idle longer. With some experience it becomes easy to tell if the engine is getting too much fuel or not enough fuel at idle. What I look for is a solid, smooth transition to full power with out a lot of hesitation or burbling. On a trainer, I'll usually take one or two burbles after a long idle, better a bit on the rich side than too lean. But on a student's plane that is about to start landing, I'll pay extra attention to the transition because I might need power in a hurry to avoid a problem.

A final note on mixtures, glow carbs are mostly pretty primative. You have a needle valve that controls fuel flow, and some kind of device to change the flow as the throttle changes. (the first is the needle valve, the second is the low speed needle). But the rate that the mixture changes though the mid-rage is a "guess" set in stone my the carb designer. It's one of the things that seperates good carbs from bad ones, and it's one of the things that makes an engine hard to tune if you run an overly large or small prop, or use a tuned pipe or do any number of other things besides run the engine as it was designed. Sometimes that means the wrong fuel or weather conditions as well.

More critically, a marginal fuel tank installation (too high or too low, or a pinch in the line), or a problem with the muffler pressure vent line or any number of other issues can often be compensated for on the high end with the needle, and at idle with the low speed, but not in the mid range. The engine either goes rich or lean in the transition because the fuel isn't being delivered quite like the designer expected.

I hope some of that helps.

Oh, and there are other ways to tune an engine. Some guys use tachs, some do a "lift test", some even try to use a thermometer to measure head temp.