If the orifices are right, then it's a gas flow issue. If you don't have enough flow coming out of the orifice, then the air intake ports won't 'suck' like they should and you end up rich.
It seems kinda backward that not enough fuel would make the jets run rich, but the system has to be pretty balanced or it doesn't perform right. Check and see if there might be some obstruction in the throat of the cast part of the burner. Either some casting slag from production or, as tends to happen around my part of the country, a dirt dobber has built himself a nest in there.