You say "gas" engines. Do you mean gasoline or glow fuel? For a gasoline engine, they can be run at a full lean setting. In fact, once set, they seldom need adjusting. Glow fuel engines are another breed. Set the needle for a full lean setting on the ground. Briefly pinch the supply fuel line. If the engine starts to die immediately, it's too lean. If it runs a while, then dies, it's too rich. If it speeds up a little, then dies, it's about right. Fly the plane. Pull a tight loop. If the engine sags at the top, it's still too lean. If you see a lot of exhaust residue, it's too rich.

Dr.1