If the engines are fit with a lot of pinch, you don't want a rich break in either, the needle should be set so that the engine just breaks into a steady 2 cycle scream with plenty of oil spray in the exhaust. The idea behind under-propping is to keep the load off and the chances of overheating down, while allowing the metals to burnish in. Once the exhaust residue runs clear, then most of the break in process is over. The low speed circuit needs to be set ONLY after the engine is throughly broke in on high speed.