Usually if the prop and spinner fly off when you put the starter to it is because the engine is flooded and backfires. No matter how much you tighten the prop nut it can still come off. One thing is to make sure the prop is not up against compression when you hit it with the starter. This will allow the piston to build up some momentum before it tries to fire and hopefully will push on through rather than backfiring. I had one OS engine that started easy and ran great but was really bad about throwing the prop off on the first start after it had not been run for awhile.