The breaking in of a new engine is important, for most engines, you want to run it fat or rich for a the first tank of fuel. During the first tank, with the prop on, run it full throttle the whole time. Every 3 minutes or so, lean it out for 20 seconds or so, then go back to fat. You want a lot of oil coming out of the exhaust. Back in the day we call them slimmers.
During the second tank, you can start to dial in the carb. Start with the high side, with the throttle wide open, slowly lean out the engine until the rpms max out, then richen the carb until you reduce the rpms to 500 below the max. Now work on the low side, the goal is to have a low idle, that can go to full throttle without hesitation. Idle the motor for 30-45 second, then pop the throttle wide open. We are simulating a landing approach, then bailing out and going around. If the engine dies immediately, the low side is lean. If the engine goes blub, blub blub, the low side is too rich. Move the low side needle valve ONLY by 1/8 turn at a time, then retest. Once you get the low side close, go back and readjust the high side and come back to the low side. During this back and forth, you should be able to lower the idle speed, an idle around 1000 rpms is good, below 1000 is great. Once you get the low side dialed in, leave it alone, you should not have to change it. The high side is different, you will need a few clicks of adjustment as the temperature changes, even a few degrees during the day.
With needle values, turning the screw in is lean (closing gap, less fuel), out is fat or rich (more fuel). There was another type carb called an air bleed, it had a small hole that let in air during the idle. With an air bleed, closing the gap reduced the air, which made the mix fat.
Their was one type of engine that you had to run lean during the first tank. I can’t remember what it was called but before it was broken in, it was extremely stiff. A normal engine will kind of bounce when you flipped the prop, this kind did not move unless you cranked it.