Most engine manuals will tell you how to tune their engines and should be followed just in case a warranty issue arises. Saito, O.S., Thunder Tigre, Super Tiger, YS, Evolution and others all have downloadable manuals on their web sites if you need one.

If you have a speific engine in mind, that will also help.

Generally, after break in, you lean the high speed needle to max rpm then richen the high speed needle about 300 to 500 rpm so you do not run lean in the air. A tach is recommended for this procedure and for a four stroke, is absolutely needed as hearing rpm changes on 4-strokes is difficult. A 2-stroke is easier to hear the changes.

Once you have the high speed set, you work on the low speed to get a smooth and rapid transition from: idle to mid-throttle and idle to max throttle. The reason for idle to mid-throttle is on most glow engines, the high speed does not really kick in until mid-throttle and above. The exception to this rule is YS engines since they use a regulator for fuel injection above roughly 2200 rpm so the low speed only works for the transition from idle to regulator and the regulator is then adjusted from there to max throttle.

On most engines to richen either needle it is turned counter-clockwise and to lean it you go clockwise. On an airbleed carb (usually on smaller engines .15 to .30 range, usually) it is usually the opposite.

One other item, the low speed needle is usually more sensitive than the high speed needle so when adjusting the low speed you start with 1/8 turns.