Check whether the key cut in the flywheel is aligned with the key cut in the shaft, that is, whether the key is present and intact. It might seem unlikely that that's your problem, but it's easy enough to check.
I have myself had trouble with kick-back when starting larger single cylinder motors if there's nothing attached, for instance, no pulley, no blade, nothing to give it momentum, and I started it too slowly. I'm getting old and can no longer quickly pull the cord on the larger engines. That is, when the motor sits by itself on the work bench, my shoulder will hurt from the kick-back, but when it's attached to whatever it's running, it starts nicely. My solution is to ask help from a young man (my Amish neighbors are helpful in this respect), who can pull the cord fast enough that there is no kick-back. It's usually an Amish motor that I'm working on anyway!