My suggestion - Could just be lower internal friction due to oil
viscosity, or even the engine simply breaking in. I'd almost be more
concerned about why it always stopped horizontally, as at least most
broken-in 4 cylinder engines stop with the prop at 45 degrees (10:30).


First verify that the mags actually do indeed shut off, then kill the
engine with the mixture, and do a hand prop to see if all cylinders
feel the same. Only if there is a noticeable difference, get a
compression check.