That's where it's important to know what kind of engine/carb. There are a couple different types and they are adjusted differently. Some have a "dual" needle design, in which case there will usually be an adjustment screw on the opposite side of the engine from the high speed adjustment screw. Most of the "cheaper" engines these days have what's called an "air bleed" carb which usually has a small hole on the front of the carb and a needle that is used to control the amount of air that enters the carb. Can you identify what you have before we get into how to set it?

Here is a link that might be helpful, if you aren't sure what engine/carb you have, could you take a couple good close up pics of the carb and engine and include in a post? it would be easier to helping you if you could.

[link]http://www.blacksheepclub.org/tuning.htm[/link]