There's a couple of them, and depending on what kind of engine you have they do different things. There's a set screw that holds the throttle body in place. On some engines this also limits the movement of the barrel. If it's an airbleed, the airbleed screw controls the lower-throttle part of the mixture. If it's a twin needle, there's a low-end needle valve. Adjust your engine purposely, rather than haphazardly.

You'll find a huge advantage in this hobby by taking the time to learn about your engine. You don't have to be an expert by any means, but doing a bit of research and reading (starting with your manual) will pay a lot of dividends. There are always people at the flying field who spend a lot of time messing with their engines. They do things like adjust the high-speed needle with the engine at idle, turn this screw or that one just to try changing something, or start messing with god knows what. Investing a few hours in theoretical knowledge and practical know-how (someone to actually show you how to properly tune an engine) will save you a lot of problems. Fewer deadsticks, fewer burned out glow plugs, and engines that do what they're supposed to do.