15 flats will almost certainly be too rich. Basic setting is considered to be 12 flats/ 1 1/2 turns down. Suck must be equal on each carb with the joining link loosened off so that each carb is adjusted seperately before locking them together and going on to mixture adjustment. ATF is too thin even if some people do use it. They too would be better off with a thicker oil. Basically use what you have in your engine, 15-40 perhaps as you have a new engine. Don't be tempted to give it too much advance. Check that your advance retard weights/springs are doing their job properly. Don't forget to oil the pad in the shaft under the rotor.
New engine settling down? Check and reset the valve clearances if needed. They need to be correct before carb adjustments. Grandma suck eggs? Just making comments...........