Mine does the same on a test stand as it does in the air. It will not hold a steady high end mixture. I have swapped carbs/needles with known good carb/needles, swapped fuel systems, RTVed the back plate, RTVed the carb, Teflon taped the needle, tried a sealed front bearing, replaced glow plugs, retorqued the head, run different fuel, experimented with tank size and height, etc, etc ad nauseam.

No big deal really, these engines are cheap enough that I can accept a lemon now and then. My previous comments were intended to help the OP, not to diss Magnum engines. I'm sure that other manufacturer's are not perfect either.