[DonC]

Olddog:
You may very well have a chip in the J3 port of the A9L supplied by Mass-Flo to adjust for whatever combinations are in the engine.
According to Tweecer, if you order the Tweecer unit and send them the J3 chip they can read it for you and create a .bin file to load into the Tweecer before you start. That way the Tweecer is providing the same information to the A9L as the J3 chip installed now. From that point you have a functioning unit and can tune from there.
For a small fee they can also read and create the .bin file from the chip without purchasing the Tweecer.
I have a Mass-Flo on my 428FE and, while the system is running well I'd still like to refine it just a bit. I'm thinking seriously of going the Tweecer R/T route this winter when the car is up for it's annual maintenance/modification/cleaning all the accumulated gluck off of it session. Like you I'm a relative novice to the EFI tuning world so it should have some "interesting" periods.
DonC