Blade plate loose.
or the key way on the flywheel.
Or the points are way out of whack.
Even if you do or don't have a compressor and a rattle gun, you can take a short length of rope and feed that into the spark plug hole and then wind the piston up against it to do the nut up.
Just take care not to poke the rope out through one of the ports then chomp it with the piston.
