If you can get it running without using starting fluid again try pulling one spark plug wire at a time and see if the knock changes sound or disappears. This will work for either a rod or piston but on a Babbitt rod 216 the rod is the weaker link. A further check is to gently rev the engine and hold at a "no load" point which is the point a rod knock usually is the loudest. We use an old Binks high pressure paint gun with gasoline in the cup and set for a very fine mist for starting engines that traditional priming doesn't work and then ONLY after the engine is already turning over. This doesn't flood the engine or wash anything out and if it backfires and ignites the spray release the trigger and flame is out.
