ORIGINAL: RoNeRiC
How do I know if it is too lean (too lean as in damaging the engine).
With the plane restrained and engine running at full speed, quickly (and carefully - watch the prop) pinch the fuel line. If the engine speeds up, you are rich. If the engine quickly dies, you are lean. With the new engine you will want the rpm to go up a little when you perform the pinch test. This will ensure that you are sufficiently rich for breakin.
Dave