OK here's what's up with that AFM. I got most of this info from Norm Garrett's book. It's a good source of Miata info straight from one of the engineers.
There is a supply voltage of 5 volts to the AFM. The output to the ECU is 5 volts at idle and it drops to 1.5 volts at WOT. It's basically a potentiometer connected to the air flap inside the thing that does this voltage drop.
The ECU sees this voltage and adjust the maps for ignition advance and injector pulse width. The speed of the motor also factors in so if you go WOT and watch the timing with a light, it travels to full advance on a curve. I would assume that the injector pulse width does a similar calculated curve.
So, if you connect the linkage of the throttle to a potentiometer and wire it up to the AFM harness, you can duplicate the action of the AFM as the throttle moves the potentiometer through it's sweep. Right now I have adapted an 86 BMW TPS to the carbs. It works pretty good but the advance curve is pretty agressive. I may wire in a trim pot and mount it on the dashboard! Nice cheap way to be able to change timing on the fly.
There's a fuel pump switch in the AFM that needs to be bypassed by connecting the two wires together. This causes the fuel pump to run whenever the key is in the run position, enging running or not!! The intake air temp sensor needs to be connected because it's output also gets factored in by the ECU. You can use a sensor from another AFM. Just take the cover off, remove the circut board and pull out the sensor. Connect the two wires and place it in the air flow near the intake.
I'm hoping that traffic will be a bit mellower today so I can go test & tune. The tourists and Christmas shopper have made the roads slow to near parade speeds