First of all, was this a new pipe or a beat up one? If it gets dented at the stinger area, it can shove the stinger back into the convergent cone and partially block it off. When this happens, motors go monster rich as the pressure has gone up.
This could also be caused by a piece of the gasket getting wedged up in there.
Or, it could be an internal manufacturing defect. One little blob of weld drips down into the wrong spot and there is the problem. No one will ever see it without cutting open the pipe.
It's not uncommon for different pipes to require vastly different tunings but this sounds extreme. Try another pipe.
Sean. Certified speed crazed mowron.
Team Shepherd USA