We just went from 1.625 to 1.875 headers on @lead69’s Dart. The engine was pretty close on tune before the headers. Just bolting on the bigger headers made the engine pig rich. And most would jump to the conclusion that the big header was killing it. In fact, the opposite was true.
The big header actually was pulling harder on the boosters and even on the idle circuit (which is exactly what you want the header to do) and made it pig rich.