I run all engines on my test stand using only two of the six 5mm bolts on any six bolt hub..Only prop I ever broke happened when the 2 bolts had bottomed out in the hub and the prop felt tight but wasn't....I run 3W 150 twins all the time.....Usually if an engine backfires it's out of time or Waaay lean......Or the ignition battery is low......
All my test props are wood, a 30-10 or a 28-12 for the biggies and a 26-10 for the smaller ones....and I ALWAYS stand behind the engine.....