It sounds like it’s time to drop the gearcase to help find the problem. It could be something simple but then again, outboard motors have been bought before with critical parts missing inside them. Hopefully that’s not the issue here.
I don’t think your Zephyr has a drive pin on the prop. There should be a large spring in there instead and a cone shaped piece. I’ll see if I can find a diagram.
EDIT: OK, this diagram shows a shear pin but I think it drives the cone, not the prop. It’s been a while since I’ve had one of these apart so I’m not sure on that. The shear pin here could be your problem.
I found a photo of the spring and a nut I made a few years ago. The spring places friction on the cone to drive the prop. You gotta have it!
