I think seat to seat duration has a direct effect regard reversion , closing intake valve too late can cause a positive pressure differential between cylinder & back of intake valve.....had reversion problems with a 440 , advanced ICL from 108* to 104* & reversion cleaned up , i believe the earlier closing int valve helped regards reversion..........same applied with the 605" , tightened intake lash .006" tighter from cam card & noticed soot around intake runners & underside of carb , loosened lash & reversion cleaned up.