Minute variations in valve adjustments, actual cylinder/ring condition, the carburetors themselves and air/fuel passage profiles as to side to side. Add it all up and one side differs from it s almost twin brother. The exhaust or the final product of combustion being correct at all throttle positions being made the same on both headers is the Holy Grail we seek. I settle for usable power at a fuel air mix that doesn't harm my engine as I'm anal enough about enough things as they are.
