Do it, $20 later you'll have either a working miata or at the very least checked one thing off the list.

Its hard to describe, but theres a happy acceleration area - too much throttle and its too lean, too little and its too rich. Feather it correctly and you find the best position (which still sucks)

The 91 has the flappy airbox, right? Thats just a potentiometer on a stick? Check the resistance open and closed.

Just an idea:
Pinch the fuel return line and give it a try.
If you have a pair of small vise-grips, use some cardboard or a towel to protect the line, and do not pinch it to the point of creating a deadhead.
If the engine seems to pick up some power that way, you could then narrow it to a FPR or the pump itself.