Check valve lash
The spring is likely missing off the compression release on the cam. Ran into the same thing this past summer.
ALL of the above,also could be a sheared key all this would cause what you have.
slap by a rope is a breeze, in the flathead days it was the whole pull starter flying back at ya lol(Break'n ropes doesn't hurt, get'n slapped by them does!)
slap by a rope is a breeze, in the flathead days it was the whole pull starter flying back at ya lol