If trying to pull-start the engine feels like there's no decompressor, I would suggest that there's too much force with which the piston is moving up. As such, the valve is closing. That could be because the engine is spinning too fast and with the force behind it of the heavy disc, when the valve closes it's like the disc/blades stopping instantly, causing the whole mower to buck. Pulling the starter slower but more consistently may be the answer, but the engine/carby would need to be functioning A1, as per the video in the previous link.
That's why I originally suggested, try starting it on level, firm ground, and note that it should start with an easy slow pull. if it's much more than that, I'd suggest the valve would close and the issue would present itself.
